Includes
Teacher's Notes
and
Typical
Experiment Results
Instruction Manual and
Experiment Guide for the
PASCO scientific Models
OS-9255A thru OS-9258A
PRECISION
INTERFEROMETER
012-07137A
5/99
© 1990 PASCO scientific $10.00
012-07137A Precision Interferometer
T able of Contents
Section Page
Copyright, Warranty, and Equipment Return...................................................ii
Introduction ......................................................................................................1
Equipment ........................................................................................................2
Theory of Operation.........................................................................................4
Michelson
Twyman-Green
Fabry-Perot
Setup and Operation.........................................................................................6
Tips on Using the Interferometer......................................................................9
Sources of Error
Troubleshooting
Experiments
Experiment 1: Introduction to Interferometry ..................................... 11
Experiment 2: The Index of Refraction of Air ...................................13
Experiment 3: The Index of Refraction of Glass ................................15
Suggestions for Additional Experiments ......................................................... 17
Maintenance ....................................................................................................18
Teacher's Guide ........................................................................................... 20-22
i
Precision Interferometer 012-07137A
Copyright, Warranty, and Equipment Return
PleaseFeel free to duplicate this manual
subject to the copyright restrictions below.
Copyright Notice
The PASCO scientific 012-05187C Precision
Interferometer manual is copyrighted and all rights
reserved. However, permission is granted to non-profit
educational institutions for reproduction of any part of
the manual providing the reproductions are used only
for their laboratories and are not sold for profit.
Reproduction under any other circumstances, without
the written consent of PASCO scientific, is prohibited.
Limited Warranty
PASCO scientific warrants the product to be free from
defects in materials and workmanship for a period of
one year from the date of shipment to the customer.
PASCO will repair or replace at its option any part of
the product which is deemed to be defective in material
or workmanship. The warranty does not cover damage
to the product caused by abuse or improper use.
Determination of whether a product failure is the result
of a manufacturing defect or improper use by the
customer shall be made solely by PASCO scientific.
Responsibility for the return of equipment for warranty
repair belongs to the customer. Equipment must be
properly packed to prevent damage and shipped postage
or freight prepaid. (Damage caused by improper
packing of the equipment for return shipment will not
be covered by the warranty.) Shipping costs for
returning the equipment after repair will be paid by
PASCO scientific.
Equipment Return
Should the product have to be returned to PASCO
scientific for any reason, notify PASCO scientific by
letter, phone, or fax BEFORE returning the product.
Upon notification, the return authorization and shipping
instructions will be promptly issued.
ä
NOTE: NO EQUIPMENT WILL BE
ACCEPTED FOR RETURN WITHOUT AN
AUTHORIZATION FROM PASCO.
When returning equipment for repair, the units must be
packed properly. Carriers will not accept responsibility
for damage caused by improper packing. To be certain
the unit will not be damaged in shipment, observe the
following rules:
1. The packing carton must be strong enough for the
item shipped.
2. Make certain there are at least two inches of packing
material between any point on the apparatus and the
inside walls of the carton.
3. Make certain that the packing material cannot shift
in the box or become compressed, allowing the
instrument come in contact with the packing carton.
Address: PASCO scientific
10101 Foothills Blvd.
Roseville, CA 95747-7100
Phone: (916) 786-3800
FAX: (916) 786-3292
email: techsupp@pasco.com
web: www.pasco.com
ii
012-07137A Precision Interferometer
Introduction
The OS-9255A Precision Interferometer provides both a
theoretical and a practical introduction to interferometry.
Precise measurements can be made in three modes:
Michelson
The Michelson Interferometer is historically important, and
also provides a simple interferometric configuration for
introducing basic principles. Students can measure the
wavelength of light and the indices of refraction of air and
other substances.
Twyman-Green
The Twyman-Green Interferometer is an important
contemporary tool for testing optical components. It has
made it possible to create optical systems that are accurate
to within a fraction of a wavelength.
➤ NOTE: The PASCO Precision Interferometer is
not designed for actual component testing in the
Twyman-Green mode. It is intended only to provide
a simple introduction to this important application of
interferometry.
Fabry-Perot
The Fabry-Perot Interferometer is also an important
contemporary tool, used most often for high resolution
spectrometry. The fringes are sharper, thinner, and more
widely spaced than the Michelson fringes, so small differences in wavelength can be accurately resolved. The
Fabry-Perot interferometer is also important in laser
theory, as it provides the resonant cavity in which light
amplification takes place.
Switching between these three modes of operation and
aligning components is relatively simple, since all mirrors
mount to the base in fixed positions, using captive panel
screws. Lenses, viewing screens, and other components
mount magnetically to the base using the included component holders.
Measurements are precise in all three modes of operation.
A 5 kg machined aluminum base provides a stable surface
for experiments and measurements. All mirrors are flat to
1/4 wavelength, and the built-in micrometer resolves mirror
movement to within one micron.
1
Precision Interferometer 012-07137A
Equipment
The OS-9255A Precision Interferometer includes the
following equipment:
5 kg Base with built-in micrometer
Adjustable Mirror
Movable Mirror
Beam Splitter
Compensator Plate
(2) Component Holder
Viewing Screen
Lens, 18 mm Focal Length
Diffuser
Fitted Storage Case
Additional Equipment Required –
Laser (OS-9171)
Laser Bench (OS-9172)
➤ NOTE: The preceding equipment includes
everything needed for basic Michelson interferometry. You can produce clear fringes and make
precise measurements of the wavelength of your
source. However, to perform the experiments in this
manual, you will need additional components, such
as the OS-9256A Interferometer Accessories or a
comparable set of your own components.The
Precision Interferometer is available as a complete
system. Please refer to your current PASCO catalog
for details.
Additional Equipment Recommended –
The OS-9256A Interferometer Accessories includes:
Rotating Pointer
Vacuum Cell
Component Holder
Lens, 18 mm Focal Length
Lens, 48 mm Focal Length
Glass Plate
(2) Polarizer
Vacuum Pump with Gauge
➤ NOTE: The OS-9255A Fitted Case also
provides storage for these accessory components.
About Your Light Source
We strongly recommend a laser for most introductory
applications. A spectral light source can be used (see the
Appendix), but that really comprises an experiment in and
of itself for beginning students. A laser source is easy to
use and produces bright, sharp fringes.
The OS-9171 Laser and OS-9172 Laser Alignment Bench
are available from PASCO. However, any low power laser
that operates in the visible range will work well. If you
want to demonstrate the importance of polarization in
interferometry, a non-polarized laser should be used. For
easy alignment, the beam should be approximately 4 cm
above the level of the bench top.
OS-9171 Laser
OS-9172
Laser Alignment
Bench
2
012-07137A Precision Interferometer
Adjustable
Mirror
(2) Component
Holder
Movable
Mirror
Fitted Case
Beam
Splitter
Lens
18 mm
Viewing
Screen
Diffuser
Compensator
Plate
Base
OS-9256A
Interferometer
Accessories
(2) Polarizer
Lens
48 mm
Glass
Plate
OS-9255A
Precision Interferometer
Lens
18 mm
Component
Holder
Rotating
Pointer
Vacuum Pump
with Gauge
Vacuum Cell
3
Precision Interferometer 012-07137A
Theory of Operation
Interference Theory
A beam of light can be modeled as a wave of oscillating
electric and magnetic fields. When two or more beams of
light meet in space, these fields add according to the
principle of superposition. That is, at each point in space,
the electric and magnetic fields are determined as the
vector sum of the fields of the separate beams.
If each beam of light originates from a separate source,
there is generally no fixed relationship between the electromagnetic oscillations in the beams. At any instant in time
there will be points in space where the fields add to
produce a maximum field strength. However, the oscillations of visible light are far faster than the human eye can
apprehend. Since there is no fixed relationship between
the oscillations, a point at which there is a maximum at one
instant may have a minimum at the next instant. The
human eye averages these results and perceives a uniform
intensity of light.
If the beams of light originate from the same source, there
is generally some degree of correlation between the
frequency and phase of the oscillations. At one point in
space the light from the beams may be continually in
phase. In this case, the combined field will always be a
maximum and a bright spot will be seen. At another point
the light from the beams may be continually out of phase
and a minima, or dark spot, will be seen.
The Michelson Interferometer
In 1881, 78 years after Young introduced his two-slit
experiment, A.A. Michelson designed and built an interferometer using a similar principle. Originally Michelson
designed his interferometer as a means to test for the
existence of the ether, a hypothesized medium in which
light propagated. Due in part to his efforts, the ether is no
longer considered a viable hypothesis. But beyond this,
Michelsons interferometer has become a widely used
instrument for measuring the wavelength of light, for using
the wavelength of a known light source to measure
extremely small distances, and for investigating optical
media.
Figure 1 shows a diagram of a Michelson interferometer.
The beam of light from the laser strikes the beam-splitter,
which reflects 50% of the incident light and transmits the
other 50%. The incident beam is therefore split into two
beams; one beam is transmitted toward the movable mirror
(M
), the other is reflected toward the fixed mirror (M2).
1
Both mirrors reflect the light directly back toward the
beam-splitter. Half the light from M1 is reflected from the
beam-splitter to the viewing screen and half the light from
M2 is transmitted through the beam-splitter to the viewing
screen.
Thomas Young was one of the first to design a method for
producing such an interference pattern. He allowed a
single, narrow beam of light to fall on two narrow, closely
spaced slits. Opposite the slits he placed a viewing screen.
Where the light from the two slits struck the screen, a
regular pattern of dark and bright bands appeared. When
first performed, Youngs experiment offered important
evidence for the wave nature of light.
Youngs slits can be used as a simple interferometer. If
the spacing between the slits is known, the spacing of the
maxima and minima can be used to determine the wavelength of the light. Conversely, if the wavelength of the
light is known, the spacing of the slits could be determined
from the interference patterns.
4
Viewing Screen
Beam
Splitter
Laser
Lens
Figure 1. Michelson Interferometer
Compensator
Adjustable Mirror
Plate
Movable Mirror
(M1)
(M2)
012-07137A Precision Interferometer
In this way the original
beam of light is split, and
➤ NOTE: Using the Compensator
portions of the resulting
beams are brought back
together. Since the
beams are from the same
source, their phases are
highly correlated. When
a lens is placed between
the laser source and the
beam-splitter, the light ray
Figure 2. Fringes
spreads out, and an
interference pattern of dark and bright rings, or fringes, is
In Figure 1, notice that one beam passes through the
glass of the beam-splitter only once, while the other
beam passes through it three times. If a highly coherent and monochromatic light source is used,
such as a laser, this is no problem. With other light
sources this is a problem.
The difference in the effective path length of the
separated beams is increased, thereby decreasing
the coherence of the beams at the viewing
screen. This will obscure the interference pattern.
seen on the viewing screen (Figure 2).
A compensator is identical to the beam-splitter, but
Since the two interfering beams of light were split from the
same initial beam, they were initially in phase. Their
relative phase when they meet at any point on the viewing
screen, therefore, depends on the difference in the length
without the reflective coating. By inserting it in the
beam path, as shown in Figure 1, both beams pass
through the same thickness of glass, eliminating this
problem.
of their optical paths in reaching that point.
By moving M
, the path length of one of the beams can be
1
varied. Since the beam traverses the path between M1 and
the beam-splitter twice, moving M1 1/4 wavelength nearer
the beam-splitter will reduce the optical path of that beam
by 1/2 wavelength. The interference pattern will change;
the radii of the maxima will be reduced so they now
occupy the position of the former minima. If M1 is moved
an additional 1/4 wavelength closer to the beam-splitter,
the radii of the maxima will again be reduced so maxima
and minima trade positions, but this new arrangement will
be indistinguishable from the original pattern.
By slowly moving the mirror a measured distance d
, and
m
counting m, the number of times the fringe pattern is
restored to its original state, the wavelength of the light (l)
can be calculated as:
2d
m
=
l
m
If the wavelength of the light is known, the same procedure can be used to measure dm.
The Twyman-Green Interferometer
The Twyman-Green Interferometer is a variation of the
Michelson Interferometer that is used to test optical
components. A lens can be tested by placing it in the beam
path, so that only one of the interfering beams passes
through the test lens (see Figure 3). Any irregularities in the
lens can be detected in the resulting interference pattern. In
particular, spherical aberration, coma, and astigmatism
show up as specific variations in the fringe pattern.
Test
Lens
Lens
Figure 3. Twyman-Green Interferometer
5
Precision Interferometer 012-07137A
The Fabry-Perot Interferometer
In the Fabry-Perot Interferometer, two partial mirrors are
aligned parallel to one another, forming a reflective cavity.
Figure 4 shows two rays of light entering such a cavity and
reflecting back and forth inside. At each reflection, part of
the beam is transmitted, splitting each incident ray into a
series of rays. Since the transmitted rays are all split from a
single incident ray, they have a constant phase relationship
(assuming a sufficiently coherent light source is used).
The phase relationship between the transmitted rays
depends on the angle at which each ray enters the cavity
and on the distance between the two mirrors. The result is
a circular fringe pattern, similar to the Michelson pattern,
but with fringes that are thinner, brighter, and more widely
spaced. The sharpness of the Fabry-Perot fringes makes it
a valuable tool in high-resolution spectrometry.
As with the Michelson Interferometer, as the movable
mirror is moved toward or away from the fixed mirror, the
fringe pattern shifts. When the mirror movement is equal
to 1/2 of the wavelength of the light source, the new fringe
pattern is identical to the original.
Partial Mirrors
Adjustable Movable
Viewing
Screen
Figure 4. Fabry-Perot Interferometer
Setup and Operation
Laser Alignment
If you are using a PASCO Laser and Laser Alignment
Bench, the setup and alignment procedure is as follows.
If you are using a different laser, the alignment proce-
dure is similar. Adjust your laser so that the beam is
approximately 4 cm above the table top. Then align
the beam as in steps 4 and 5, below.
If you are using a spectral light source instead of a
laser, see Suggestions for Additional Experiments,
near the end of the manual.
To set up and align your PASCO Laser:
1. Set the interferometer base on a lab table with the mi-
crometer knob pointing toward you.
2. Position the laser alignment bench to the left of the
base approximately perpendicular to the interferometer
base and place the laser on the bench.
3. Secure the movable mirror in the recessed hole in the
interferometer base.
4. Turn the laser on. Using the leveling screws on the la-
ser bench, adjust its height until the laser beam is approximately parallel with the top of the interferometer
base and strikes the movable mirror in the center. (To
check that the beam is parallel with the base, place a
piece of paper in the beam path, with the edge of the
paper flush against the base. Mark the height of the
beam on the paper. Using the piece of paper, check that
the beam height is the same at both ends of the bench.)
5. Adjust the X-Y position of the laser until the beam is
reflected from the movable mirror right back into the
laser aperture. This is most easily done by gently sliding the rear end of the laser transverse to the axis of the
alignment bench, as shown in Figure 5.
You are now ready to set up the interferometer in any of
its three modes of operation.
6
012-07137A Precision Interferometer
N
E
Laser beam
VIEWING SCREEN
MICHELSON, TWYMAN-GREEN
N
R
O
E
T
T
18 mm FL LENS
R
S
O
L
I
T
E
L
A
H
N
P
S
C
S
O
I
N
S
E
M
M
L
P
A
E
M
E
H
B
O
C
I
C
M
Movable
mirror
Slide the rear of the
laser laterally on the
alignment bench until
ADJUSTABLE MIRROR
MICHELSON, TWYMAN-GREEN
25
5
0
15
-5
10
the beam is reflected
straight back into the
laser aperture.
Figure 5. Aligning the Laser
➤ NOTE:
For ease of installation the placement of the individual
components in the various modes is indicated on the
label.
Component
holder
VIEWING SCREEN
OS-9255A
MICHELSON, TWYMAN-GREEN
Viewing screen
MICHELSON
30
Laser
18 mm FL LENS
Component
holder
BEAM SPLITTER
Beam
splitter
Lens
Interferometer
base
INTERFEROM
Compensator
(optional)
COMPENSATOR
MICHELSON
Movable
mirror
Adjustable mirror
PRECISIO
18mm FL
ADJUSTABLE MIRROR
MICHELSON, TWYMAN-GREEN
5
0
-5
30
25
15
10
1 div =
Thumbscrews
Michelson Mode
1. Align the laser and interferometer base as previously
described. The laser beam should be approximately
parallel with the top of the base, should strike the center
of the movable mirror, and should be reflected directly
back into the laser aperture.
2. Mount the adjustable mirror on the interferometer base.
Position one component holder in front of the laser.
Place the other component holder opposite the adjustable mirror and attach the viewing screen to its magnetic backing. See Figure 6.
3. Position the beam-splitter at a 45 degree angle to the
laser beam, within the crop marks, so that the beam is
reflected to the fixed mirror. Adjust the angle of the
beam-splitter as needed so that the reflected beam hits
the fixed mirror near its center.
4. There should now be two sets of bright dots on the
viewing screen; one set comes from the fixed mirror
and the other comes from the movable mirror. Each
set of dots should include a bright dot with two or more
dots of lesser brightness (due to multiple reflections).
Adjust the angle of the beam-splitter again until the two
sets of dots are as close together as possible, then
tighten the thumbscrew to secure the beam-splitter.
Micrometer
knob
Figure 6. Michelson Mode Setup
5. Using the thumbscrews on the back of the adjustable
mirror, adjust the mirrors tilt until the two sets of dots
on the viewing screen coincide.
6. The compensator is not needed for producing interfer-
ence fringes when using a laser light source. However,
if you wish to use the compensator, it mounts perpendicular to the beam-splitter, as shown.
7. Attach the 18 mm FL lens to the magnetic backing of
the component holder in front of the laser, as shown,
and adjust its position until the diverging beam is centered on the beam-splitter. You should now see circular fringes on the viewing screen. If not, carefully adjust the tilt of the adjustable mirror until the fringes appear.
8. If you have trouble obtaining fringes, see Trouble-
Shooting at the end of this section.
7
Precision Interferometer 012-07137A
Twyman-Green Mode
1. Set up the interferometer in the Michelson mode, as
described above.
2. Remove the pointer from the rotational componet
holder. (It is recommended to store the pointer, washer
and thumbscrew in the storage case.) Place the component holder between the beam-splitter and the movable
mirror (see Figure 7). It attaches magnetically. Mount a
second 18 mm FL lens (L
) on its magnetic backing
2
and position it .
3. Remove the original lens (L
) from in front of the laser.
1
Observe the two sets of dots on the viewing screen
one set from the movable mirror and one set from the
adjustable mirror. Adjust the position of L
until both
2
sets of dots are the same size.
4. Adjust the tilt of the adjustable mirror until the two sets
of dots coincide.
5. Replace lens L
screen so it's at least 12 inches from the edge of the
interferometer base. Fringes should appear in the bright
disk of the viewing screen. Fine adjustments of L
may be necessary to find the fringes. A piece of white
paper or cardboard can be used in place of the viewing
screen. A 48 mm FL convex lens may also be used to
magnify the projected image of the fringes.
in front of the laser. Move the viewing
1
1
Fabry-Perot Mode
1. Align the laser and interferometer base as described in
Laser Alignment at the beginning of this section. The
laser beam should be approximately parallel with the
top of the base, should strike the center of the movable
mirror, and should be reflected directly back into the
laser aperture.
2. Mount the adjustable mirror where indicated on the in-
terferometer base and one component holder in front of
the movable mirror. See Figure 8.
3. Place the other component holder behind the movable
mirror and attach the viewing screen to its magnetic
backing. You should see several images of the laser
beam on the viewing screen.
4. Using the thumbscrews, adjust the tilt of the adjustable
mirror until there is only one bright dot on the screen.
5. Now mount the 18 mm FL lens on the front compo-
nent holder. A clear sharp interference pattern should
be visible on the viewing screen. If you use light with
two component wavelengths, instead of a laser, two sets
of fringes can be distinguished on the viewing screen.
Lens
18mm FL
OS-9255A
Adjustable mirror
VIEWING SCREEN
MICHELSON, TWYMAN-GREEN
Movable mirror
PRECISION
INTERFEROMETER
VIEWING SCREEN
OS-9255A
18 mm FL LENS
MICHELSON, TWYMAN-GREEN
N
R
O
E
S
T
L
IT
E
L
H
P
IC
S
M
M
A
E
B
R
O
T
A
N
S
O
N
S
E
L
P
E
M
H
O
IC
C
M
PRECISION
INTERFEROMETER
Lens
18mm FL
18 mm FL LENS
Component
holder
N
LSO
E
LITTER
ICH
SP
M
ICHELSON
COMPENSATOR
M
BEAM
Viewing screen
Component
holder
Rotational
ADJUSTABLE MIRROR
MICHELSON, TWYMAN-GREEN
component holder
30
25
5
0
-5
15
10
1 div = 1 MICRON
ADJUSTABLE MIRROR
MICHELSON, TWYMAN-GREEN
5
0
-5
30
25
15
10
1 div = 1 MICRON
Figure 8. Fabry-Perot Mode Setup
Figure 7. Twyman-Green Mode Setup
8
012-07137A Precision Interferometer
Tips on Using the Interferometer
Accurate Fringe-Counting
The following techniques can help you make accurate
measurements.
1. It's not necessary that your interference pattern be per-
fectly symmetrical or sharp. As long as you can clearly
distinguish the maxima and minima, you can make accurate measurements.
2. It's easy to lose track when counting fringes. The fol-
lowing technique can help.
Center the interference
pattern on the viewing
screen using the thumbscrews on the back of the
fixed mirror. Select a reference line on the millimeter scale and line it up
with the boundary between a maxima and a
minima (see Figure 9).
Move the micrometer dial
until the boundary between the next maximum and
minimum reaches the same position as the original
boundary. (The fringe pattern should look the same as
in the original position.) One fringe has gone by.
Figure 9.
Counting Fringes
5. The slip ring at the base of the micrometer knob adjusts
the tension in the dial. Before making a measurement,
be sure the tension is adjusted to give you the best possible control over the mirror movement.
Calibrating the Micrometer
For even more accurate measurements of the mirror
movement, you can use a laser to calibrate the micrometer.
To do this, set up the interferometer in Michelson or
Fabry-Perot mode. Turn the micrometer knob as you
count off at least 20 fringes. Carefully note the change in
the micrometer reading, and record this value as d'. The
actual mirror movement, d, is equal to Nl/2, where l is
the known wavelength of the light (0.6328 µm for a
standard helium-neon laser) and N is the number of fringes
that were counted. In future measurements, multiply your
micrometer readings by d/d' for a more accurate measurement.
ä
NOTE: You can also adjust the micrometer
calibration mechanically. The process is not difficult,
but for most accurate results, the above procedure is
still recommended. See the Maintenance section at
the end of the manual for the mechanical calibration
procedure.
3. When turning the micrometer dial to count fringes, al-
ways turn it one complete revolution before you start
counting, then continue turning it in the same direction
while counting. This will almost entirely eliminate errors due to backlash in the micrometer movement.
Backlash is a slight slippage that always occurs when
you reverse the direction of motion in a mechanical instrument. (Turning the micrometer dial clockwise
moves the movable mirror toward the right. Turning
the dial counter-clockwise moves the mirror toward the
left.) The PASCO micrometer is designed to minimize
backlash. However, by using the technique described
above, you can practically eliminate all effects of backlash in your measurements.
4. Always take several readings and average them for
greater accuracy.
Demonstrations
The PASCO interferometer is not designed for large
demonstrations. However, for small demonstrations, you
can use the 48 mm focal length lens (included in the
Interferometer Accessories) to magnify the fringe pattern
and project it onto a wall or screen. It is helpful to have a
powerful laser for large projections.
Using the Diffuser
It's sometimes more convenient to view the interference
pattern through the diffuser rather than on the viewing
screen. Just place the diffuser where you would normally
place the viewing screen, and look through it toward the
interferometer.
9
Precision Interferometer 012-07137A
Sources of Experimental Error
Backlash Although PASCO's carefully designed
mirror movement reduces backlash considerably, every
mechanical system is susceptible to backlash. However,
the effects of backlash can be practically eliminated by
using proper technique when counting fringes (see item 3
under Accurate Fringe-Counting, on the previous page).
Mirror Travel The amount of mirror movement per
dial turn of the micrometer is constant to within 1.5%.
Most of this error occurs at the extreme ends of the
mirrors total possible movement. For very accurate
measurements, see Calibrating the Micrometer, above,
and remember that the mirrors are flat to within 1/4
wavelength across their surface.
Troubleshooting
If you have trouble producing a clear set of interference
fringes, consider the following possible sources of difficulty:
1. Warm up your Laser Many lasers vary in intensity
and/or polarization as they warm up. To eliminate any
possible fringe or intensity variations, allow the laser to
warm up prior to setting up an experiment. (The
PASCO laser should warm up in about 1 hour.)
2. Check your Mirrors The beam-splitter and
movable mirror are carefully mounted in their brackets
to remain perpendicular to the interferometer base
when set up. If the brackets are bent slightly out of
alignment, the resulting fringe patterns will be distorted
somewhat. If they are significantly out of alignment, it
may be impossible to obtain fringes.
ä
IMPORTANT: If the movable mirror doesn't
move when you turn the micrometer dial, see
Micrometer Spacer Replacement in the
Maintenance section at the end of this manual.
Component Specifications
Interferometer Mirrors 3.175 cm in diameter;
0.635 + 0.012 cm thick; flat to 1/4 wavelength on both
sides; coated on one side for 80% reflectance and 20%
transmission.
Beam-Splitter 3.175 cm in diameter; 0.635
thick; flat to 1/4 wavelength on both sides; coated on one
side for 50% reflectance and 50% transmission.
Compensator Identical to the beam-splitter, but
uncoated.
Movable Mirror movement is controlled by the
micrometer that is built-into the interferometer base;
turning the dial clockwise moves the mirror toward the
right (looking from the micrometer side); 25 microns per
micrometer dial revolution (±1% near center of movement); movement through full distance of travel is linear to
within 1.5%.
ä
IMPORTANT: Avoid touching all mirror
surfaces. Minute scratches and dirt can impair the
clarity of interference images. See the Maintenance
section at the end of this manual for cleaning
instructions.
+ 0.012 cm
3. Background Fringes Reflections from the front
and back surfaces of the mirrors and beam-splitter often cause minor interference patterns in the background of the main fringe pattern. These background
patterns normally do not move when the mirror is
moved, and have no impact on measurements made
using the main interference pattern.
4. Convection Currents If the fringe pattern ap-
pears to wave or vibrate, check for air currents. Even
a slight breeze can effect the fringes.
5. Vibration Under normal conditions, the
interferometer base and mirror mounts are stable
enough to provide a vibration free setup. However, if
the experiment table is vibrating sufficiently, it will
effect the interference pattern.
10
012-07137A Precision Interferometer
Experiment 1: Introduction to Interferometry
EQUIPMENT NEEDED:
Basic Interferometer (OS-9255A)
Laser (OS-9171)
Laser Alignment Bench (OS-9172)
Interferometer Accessories (OS-9256A)
Component Holder , (2) Calibrated Polarizers
Introduction
Lens
18mm FL
Component holder
OS-9255A
VIEWING SCREEN
MICHELSON, TWYMAN-GREEN
Viewing screen
PRECISION
INTERFEROMETER
In general, an interferometer can be used in
two ways. If the characteristics of the light
source are accurately known (wavelength,
polarization, intensity), changes in the beam
path can be introduced and the effects on the
18 mm FL LENS
splitter
Beam
MICHELSON
BEAM SPLITTER
COMPENSATOR
MICHELSON
Compensator
(optional)
Movable
interference pattern can be analyzed. Experiments 2 and 3 are examples of this procedure. On
the other hand, by introducing specific changes in
the beam path, information can be obtained
ADJUSTABLE MIRROR
MICHELSON, TWYMAN-GREEN
5
0
-5
15
10
about the light source that is being used.
In this experiment, you'll use the interferometer to
measure the wavelength of your light source. If
you have a pair of polarizers, you can also
investigate the polarization of your source.
Procedure
Figure 1.1. Michelson Mode Setup
Adjustment
Thumbscrews
Micrometer
knob
Adjustable
mirror
Part I: Wavelength
1. Align the laser and interferometer in the Michelson mode, so an interference pattern is clearly visible on your
viewing screen. See Setup and Operation for instructions.
2. Adjust the micrometer knob to a medium reading (approximately 50 µm). In this position, the relationship be-
tween the micrometer reading and the mirror movement is most nearly linear.
mirror
30
25
1 div = 1 MICRON
3. Turn the micrometer knob one full turn counterclockwise. Continue turning counterclockwise until the zero on the
knob is aligned with the index mark. Record the micrometer reading.
➤ NOTE: When you reverse the direction in which you turn the micrometer knob, there is a small amount of
give before the mirror begins to move. This is called mechanical backlash, and is present in all mechanical
systems involving reversals in direction of movement. By beginning with a full counterclockwise turn, and then
turning only counterclockwise when counting fringes, you can eliminate errors due to backlash.
4. Adjust the position of the viewing screen so that one of the marks on the millimeter scale is aligned with one of the
fringes in your interference pattern. You will find it easier to count the fringes if the reference mark is one or two
fringes out from the center of the pattern.
5. Rotate the micrometer knob slowly counterclockwise. Count the fringes as they pass your reference mark.
Continue until some predetermined number of fringes have passed your mark (count at least 20 fringes). As you
finish your count, the fringes should be in the same position with respect to your reference mark as they were
when you started to count. Record the final reading of the micrometer dial.
11
Precision Interferometer 012-07137A
6. Record d
, the distance that the movable mirror moved toward the beam-splitter according to your
m
readings of the micrometer knob. Remember, each small division on the micrometer knob corre-
6
-
sponds to one µm (10
meters) of mirror movement.
7. Record N, the number of fringe transitions that you counted.
8. Repeat steps 3 through 7 several times, recording your results each time.
9. Go on to part two. If you have time afterward, try setting up the interferometer in Fabry-Perot mode
and repeating steps 3 through 8.
Part II : Polarization (using the Calibrated Polarizer, part of OS-9256A Interferometer Accessories)
1. Place a polarizer between the laser and the beam-splitter. Try several polarization angles. How does
this effect the brightness and clarity of the fringe pattern?
2. Remove that polarizer and place a polarizer in front of the fixed or movable mirror. Try several
polarization angles. How does this effect the fringe pattern?
3. Now try two polarizers, one in front of the fixed mirror, and one in front of the movable mirror. First
rotate one polarizer, then the other. Again, note the effects.
Analysis
Part I
1. For each trial, calculate the wavelength of the light (l = 2d
the Fabry-Perot mode also, calculate the wavelength independently for that data. The same formula
applies.
/N), then average your results. If you tried
m
Part II
1. From your observations in step 1 of the procedure, can you determine the polarization characteristics
of your light source? Does it vary with time?
2. Do your observations from step 2 give you any more information about the polarization of your
source?
3. From your observations in step 3, do cross-polarized beams interfere?
Questions
1. In the calculation to determine the value of l based on the micrometer movement, why was d
multiplied by two?
2. Why move the mirror through many fringe transitions instead of just one? Why take several measure-
ments and average the results?
3. If you tried the Fabry-Perot mode, was your measured l the same? If not, can you speculate about possible
reasons for the difference? Do you have more confidence in one value as opposed to the other?
4. If the wavelength of your light source is accurately known, compare your results with the known
value. If there is a difference, to what do you attribute it?
5. When measuring mirror movement using the micrometer dial on the interferometer, what factors limit
the accuracy of your measurement?
m
6. When measuring mirror movement by counting fringes using a light source of known wavelength,
what factors might limit the accuracy of your measurement?
7. What role does polarization play in producing an interference pattern?
12
012-07137A Precision Interferometer
Experiment 2: The Index of Refraction of Air
EQUIPMENT NEEDED:
Basic Interferometer (OS-9255A)
Laser (OS-9171)
Laser Alignment Bench (OS-9172)
Interferometer Accessories (OS-9256A)
Rotational pointer, Vacuum cell, Vacuum pump
Introduction
2
1
In the Michelson interferometer, the characteristics of the
fringe pattern depend on the phase relationships between
the two interfering beams. There are two ways to change
the phase relationships. One way is to change the distance
traveled by one or both beams (by moving the movable
mirror, for example). Another way is to change the
medium through which one or both of the beams pass.
Either method will influence the interference pattern. In
this experiment you will use the second method to
measure the index of refraction for air.
For light of a specific frequency, the wavelength l varies
according to the formula:
l = lo/n;
where lo is the wavelength of the light in a vacuum, and
n is the index of refraction for the material in which the
light is propagating. For reasonably low pressures, the
index of refraction for a gas varies linearly with the gas
pressure. Of course for a vacuum, where the pressure is
zero, the index of refraction is exactly 1. A graph of
index of refraction versus pressure for a gas is shown in
Figure 2.1. By experimentally determining the slope, the
index of refraction of air can be determined at various
pressures.
Index of Refraction (n)
0
0
Gas Pressure (cm Hg)
Figure 2.1. Index of Refraction versus Gas
Pressure
VIEWING SCREEN
OS-9255A
18 mm FL LENS
MICHELSON, TWYMAN-GREEN
ADJUSTABLE MIRROR
MICHELSON, TWYMAN-GREEN
MICHELSON
BEAM SPLITTER
COMPENSATOR
MICHELSON
0
-5
PRECISION
INTERFEROMETER
Vacuum Cell
Air Outlet
30
25
5
15
10
1 div = 1 MICRON
Figure 2.2. Equipment Setup
Procedure
1. Align the laser and interferometer in the Michelson mode. See Setup and Operation.
2. Place the rotational pointer between the movable mirror and the beam-splitter (see Figure 2.2).
Attach the vacuum cell to its magnetic backing and push the air hose of the vacuum pump over the
air outlet hole of the cell. Adjust the alignment of the fixed mirror as needed so the center of the
interference pattern is clearly visible on the viewing screen. (The fringe pattern will be somewhat
distorted by irregularities in the glass end-plates of the vacuum cell. This is not a problem.)
3. For accurate measurements, the end-plates of the vacuum cell must be perpendicular to the laser
beam. Rotate the cell and observe the fringes. Based on your observations, how can you be sure
that the vacuum cell is properly aligned?
13
Precision Interferometer 012-07137A
λ
4. Be sure that the air in the vacuum cell is at atmospheric pressure. If you are using the OS-8502 Hand-
Held Vacuum Pump, this is accomplished by flipping the vacuum release toggle switch.
5. Record P
, the initial reading on the vacuum pump gauge. Slowly pump out the air in the vacuum cell. As
i
you do this, count N, the number of fringe transitions that occur. When you're done, record N and also
P
, the final reading on the vacuum gauge. (Some people prefer to begin with the vacuum cell evacuated,
f
then count fringes as they let the air slowly out. Use whichever method is easier for you.)
➤ NOTE: Most vacuum gauges measure pressure with respect to atmospheric pressure (i.e., 34 cm Hg
means that the pressure is 34 cm Hg below atmospheric pressure, which is ~ 76 cm Hg). The actual
pressure inside the cell is:
Analyzing Your Data
As the laser beam passes back and forth between the beam-splitter and the movable mirror, it passes
twice through the vacuum cell. Outside the cell the optical path lengths of the two interferometer beams
do not change throughout the experiment. Inside the cell, however, the wavelength of the light gets longer
as the pressure is reduced.
Suppose that originally the cell length, d, was 10 wavelengths long (of course, it's much longer). As you
pump out the cell, the wavelength increases until, at some point, the cell is only 9-1/2 wavelengths long.
Since the laser beam passes twice through the cell, the light now goes through one less oscillation within
the cell. This has the same effect on the interference pattern as when the movable mirror is moved
toward the beam-splitter by 1/2 wavelength. A single fringe transition will have occurred.
Originally there are N
beam). At the final pressure there are Nf = 2d/lf wavelengths within the cell. The difference between
these values, Ni Nf , is just N, the number of fringes you counted as you evacuated the cell. Therefore:
N = 2d/li - 2d/lf.
P
= P
absolute
= 2d/li wavelengths of light within the cell (counting both passes of the laser
i
atmospheric
– P
gauge
However, l
= l0/ni and lf = l0/nf; where ni and nf are the initial and final values for the index of
i
refraction of the air inside the cell. Therefore N = 2d(ni nf) /l0; so that ni nf = Nl0/2d. The slope of
the n vs pressure graph is therefore:
ni– n
Pi–P
f
f
N
=
2d(Pi–Pf)
0
where Pi = the initial air pressure; Pf = the final air pressure; ni = the index of refraction of air at pressure
Pi; nf = the index of refraction of air at pressure Pf ; N = the number of fringe transitions counted during
evacuation; l0 = the wavelength of the laser light in vacuum (see your instructor);
d = the length of the vacuum cell (3.0 cm).
1. Calculate the slope of the n vs pressure graph for air.
2. On a separate piece of paper, draw the n vs pressure graph.
Questions
1. From your graph, what is n
, the index of refraction for air at a pressure of 1 atmosphere (76 cm Hg).
atm
2. In this experiment, a linear relationship between pressure and index of refraction was assumed. How
might you test that assumption?
3. The index of refraction for a gas depends on temperature as well as pressure. Describe an experiment
that would determine the temperature dependence of the index of refraction for air.
14
012-07137A Precision Interferometer
OS-9255A
PRECISION
INTERFEROMETER
-5
0
5
10
15
25
30
ADJUSTABLE MIRROR
MICHELSON, TWYMAN-GREEN
B
E
A
M
S
P
L
I
T
T
E
R
M
I
C
H
E
L
S
O
N
C
O
M
P
E
N
S
A
T
O
R
M
I
C
H
E
L
S
O
N
18 mm FL LENS
VIEWING SCREEN
MICHELSON, TWYMAN-GREEN
1 div = 1 MICRON
Experiment 3: The Index of Refraction of Glass
EQUIPMENT NEEDED:
Basic Interferometer (OS-9255A)
Laser (OS-9171)
Laser Alignment Bench (OS-9172)
Interferometer Accessories
Rotating Table, Glass Plate
Introduction
In Experiment 2, the index of refraction of air was
measured by slowly varying the density of air along a
fixed length of one beam path in the Michelson
Interferometer. That method obviously won't work
with a solid substance, such as glass. Therefore, in
order to measure the index of refraction of glass, it's
necessary to slowly vary the length of glass through
which the interferometer beam passes. This experiment introduces a technique for making such a
measurement.
Procedure
Glass plate
Rotational
pointer
Read Angle of
Inclination on
Degree Scale
1. Align the laser and interferometer in the Michelson
mode. See Setup and Operation.
Figure 3.1. Equipment Setup
2. Place the rotating table between the beam-splitter
and movable mirror, perpendicular to the optical path.
➤ NOTE: if the movable mirror is too far forward, the rotating table won't fit. You may need to
loosen the thumbscrew and slide the mirror farther back.
3. Mount the glass plate on the magnetic backing of the rotational pointer.
4. Position the pointer so that its 0 edge on the Vernier scale is lined up with the zero on the degree
scale on the interferometer base.
5. Remove the lens from in front of the laser. Hold the viewing screen between the glass plate and
the movable mirror. If there is one bright dot and some secondary dots on the viewing screen,
adjust the angle of the rotating table until there is one bright dot. Then realign the pointer scale.
The plate should now be perpendicular to the optical path.
6. Replace the viewing screen and the lens and make any minor adjustments that are necessary to get
a clear set of fringes on the viewing screen.
7. Slowly rotate the table by moving the lever arm. Count the number of fringe transitions that occur
as you rotate the table from 0 degrees to an angle q (at least 10 degrees).
15
Precision Interferometer 012-07137A
λ
Data Analysis
In principle, the method for calculating the index of refraction is relatively simple. The light passes
through a greater length of glass as the plate is rotated. The general steps for measuring the index of
refraction in such a case is as follows:
1. Determine the change in the path length of the light beam as the glass plate is rotated. Determine how
much of the change in path length is through glass, d
2. Relate the change in path length to your measured fringe transitions with the following equation:
2nada(θ)+2ngdg(θ)
λ
where n
= the index of refraction of air (see Experiment 2), ng = the index of refraction of the glass
a
plate (as yet unknown), l0 = the wavelength of your light source in vacuum, and N = the number of
fringe transitions that you counted.
Carrying out this analysis for the glass plate is rather complicated, so we'll leave you with the equation
shown below for calculating the index of refraction based on your measurements. Nevertheless, we
encourage you to attempt the analysis for yourself. It will greatly increase your understanding of the
measurement and also of the complications inherent in the analysis.
(q), and how much is through air, da(q).
g
0
(2t – N
2t(1 – cos θ)–Nλ
)(1 – cosθ)
0
0
where t = the thickness of the glass plate.
➤ NOTE: Our thanks to Prof. Ernest Henninger, DePauw University, for providing this equation
from Light Principles and Measurements, by Monk, McGraw-Hill, 1937.
16
012-07137A Precision Interferometer
Suggestions for Additional Experiments
Twyman-Green
Twyman-Green operation gives students a quick, qualitative look at how interferometry can be used to test optical
components. See Twyman-Green Mode in the Setup and
Operation section of the manual.
Any distortion of the circular fringe pattern is due to
spherical aberration from the test lens. Turn the lens until
it sits at various angles to the optical path and watch the
fringe pattern change. Distortion here is due partially to
astigmatism from the lens.
Spectral Light Fringes
Although interferometry is easiest with a laser light source,
measurements can be made successfully using any monochromatic source of sufficient brightness. However, if a
laser is not used, it is generally not possible to project the
interference fringes onto a screen. Instead, the fringes are
viewed by looking into the beam-splitter (or into the
movable mirror in Fabry-Perot mode).
If you use a spectral light source with spectral lines at
several different frequencies, it may be necessary to use a
filter that blocks all but one of the spectral wavelengths.
Michelson Mode:
ä
NOTE:
One difficulty when using a non-laser light source in
Michelson mode is that the coherence length of the
light is far less with a non-laser source. Because of
this, the compensator should be used. It mounts
magnetically on the back of the beam-splitter (the
side opposite the thumbscrew).
It's also important that the optical paths of the two
interfering beams should be nearly equal. To ensure
that this is the case, set up the interferometer with a
laser (if you have one) and adjust the movable
mirror position until the fewest possible fringes
appear on the screen. (Theoretically, when the beam
paths are exactly equal, one big maximum should
appear that occupies the whole screen. But this is
usually not possible to achieve in practice due to
optical imperfections.) Then remove the viewing
screen and replace the laser with the spectral light
source. If fringes aren't visible when looking into the
beam-splitter, proceed as follows:
a. Tape two thin pieces of wire or thread to the surface of
the diffuser to form cross-hairs.
b. Place the diffuser between the light source and the
beam-splitter.
c. Adjust the angle of the beam-splitter so that, when look-
ing into the beam-splitter, you can see two images of
the cross-hairs.
d. Adjust the tilt of the fixed mirror until the cross-hairs are
superimposed. You should be able to see the fringe pattern.
Fabry-Perot mode:
a. Tape two thin pieces of wire or thread to the surface of
the diffuser to form cross-hairs.
b. Set up the equipment in Fabry-Perot mode, and place
the diffuser between the light source and the fixed mirror.
c. Look into the movable mirror from behind. Adjust the
tilt of the fixed mirror until the cross-hairs are superimposed. You should be able to see the fringe pattern.
White Light Fringes
With careful alignment, the interferometer will produce
fringes from multi-chromatic or even white light . The
procedure is the same as for any non-laser source, as
described above. However, since it is harder to get a visible
interference pattern, it is strongly recommended that you
first set up the interferometer using a laser. Then substitute
your white light source.
Use a Photometer
Use a photometer, such as PASCO Model OS-9152B, to
scan the fringe patterns. You can compare the intensity
distributions in the Michelson and Fabry-Perot modes. Or
use it to more accurately determine polarization effects. Or
just use it as an aid in counting fringes.
Heat Distribution in Air
With the interferometer in Michelson mode, strike a match
and bring it close to one of the optical paths. Note the
distortions in the fringe pattern. For a more quantitative
approach, you could construct an air tight cell, and heat the
contents to observe the effects of heat on the index of
refraction of air.
17
Precision Interferometer 012-07137A
ä
IMPORTANT The Vacuum Cell is not designed to
be heated.
Index of Refraction for Gases
Measure the indices of refraction for various gases.
Caution: The PASCO Vacuum Chamber is NOT designed
Fabry-Perot Spectroscopy
The Fabry-Perot mode is customarily used as a highresolution spectrometer. Very close spectral lines, as in
magnetic splitting, can be resolved much more accurately
than with any but the highest quality diffraction gratings.
to hold positive pressures. You will need to provide your
own gas chamber.
Maintenance
Micrometer Calibration
The micrometer is calibrated before it is shipped. However, if recalibration becomes necessary, use the following
procedure:
1. Turn the interferometer
over, and remove the bottom cover.
2. Loosen the two screws
shown in Figure A1. Slide
the bearing surface toward
the pivot to increase mirror movement per turn of
the micrometer dial. Slide
the bearing surface away
from the pivot to decrease
mirror movement per dial
turn. Tighten the screws
and replace the bottom
cover.
Testing your calibration is most easily performed using a
laser light source of known wavelength, as in
Experiment 1.
Loosen
screws and
slide the
bearing
surface as
required.
Figure A1.
Calibration
To replace the spacer:
1. Turn the interferometer
over, and remove the bottom cover.
Spacer
2. Position the spacer be-
tween the two ball bearings, as shown in Figure
A2. Release the lever, and
check that the spacer is
snugly in place.
3. Replace the bottom panel.
Lever
arm
Figure A2. Spacer
Replacement
Mirror Care
The mirror and beam-splitter surfaces are precision ground
and coated. Dirt or scratches will distort the fringe pattern,
so handle all optical surfaces with care. Clean the surfaces
occasionally with lens tissue.
Vacuum Cell
Clean the glass windows on the vacuum chamber occasionally with lens tissue.
Micrometer Spacer Replacement
In order to provide extremely fine, backlash-free control of
the movable mirror, the mechanical linkage between the
micrometer and the movable mirror is maintained under a
state of spring-loaded compression. This compression also
holds part of the linkage (a spacer) in place. Under normal use,
the spacer will never fall out of position. However, a sudden
jolt can jar the spacer and the spring loose. In this case, the
micrometer will no longer work, and you'll hear the parts
rolling around inside.
18
Storage
Rotate the Micrometer Knob fully IN before storing the
Interferometer.
012-07137A Precision Interferometer
Replacement Parts
Component Part No.
Interferometer Base 003-05137
Adjustable Mirror 003-03957
Beam-Splitter 003-03956
Movable Mirror 003-03955
Component Holder 003-05161
Compensator 003-03958
Interferometer Manual 012-05187
Vacuum Pump OS-8502
Component Part No.
Vacuum Cell 003-05162
Rotational Pointer 003-05160
Fitted Case 650-05178
Viewing Screen 003-05119
Diffuser 003-03941
Polarizer 003-04924
Glass Plate 003-04034
Lens, 18mm FL 003-03814
Lens, 48mm FL 003-03806
19
Precision Interferometer 012-07137A
Teacher's Guide
Experiment 1: Introduction to Interferometry
Part I General
Dm wavelength
Michelson 1.60 x 10
1.60 x 10
1.60 x 10
Fabry-Perot 1.60 x 10
1.50 x 10
1.55 x 10
average: 630.0 x 109±16.7 x 10
actual: 632.8 x 10
5
5
5
5
5
5
640.0 x 10
640.0 x 10
640.0 x 10
640.0 x 10
600.0 x 10
620.0 x 10
9
9
9
9
9
9
9
9
% diff. 0.44%
Part II General
1. The pattern became somewhat dimmer, due to
absorbtion by the polarizer; but other than that, there
was no variation when we polarized the light coming
into the interferometer.
2. Adding a polarizer in front of the movable mirror had
little effect. The contrast of the interference pattern
reduced, and the pattern rotated when the polarizer was
rotated.
3. There was no pattern unless the two polarizers were in
the same orientation.
Reference to Analysis (Part II)
1. The laser we used was unpolarized, and does not seem
to change polarization with time.
2. No, there was no change. This would support our
hypothesis that the laser used was unpolarized.
3. Cross-polarized beams do not interfere.
Answers to Questions
1. The change in path length is twice the movement of the
mirror.
2. Measuring only many fringes, many times, decreases
the chance of random error affecting our results.
3. They were roughly the same. The Fabry-Perot
measurement could instill more confidence, because the
fringes are sharper and easier to count.
4. The difference is probably due to our uncertainty in
measurement.
5. Limiting factors are play in the system and uncertainty
in our micrometer position.
6. Losing count of fringes, and inexact positioning of the
fringes relative to our reference mark.
7. In order to interfere, the two light beams must have the
same polarization.
20
012-07137A Precision Interferometer
Experiment 2: The Index of Refraction of Air
Reference to Procedure
1. The chamber will be properly aligned when the reflec-
tions off the front and back end-plates are aligned with
each other and with the main interference pattern. (This
alignment may actually cause a secondary interference
pattern, but it will be very faint and will not affect your
measurements.)
2. It seems easiest to apply the vacuum first, then count
the fringes as the vacuum was released.
The average slope, starting at a guage reading of 60,
was 3.462x10
-6
.
1.0005
1.0004
1.0003
1.0002
1.0001
1
0.9999
0.9998
0.9997
0.9996
0.9995
0 102030405060708090
Pressure (cm hg)
Answers to Questions
1. Extrapolating from our slope and the known index of
refraction of vacuum,
n
= 1.000263.
atm
2. Measure the index of refraction at various pressures,
and see if it increases linearly. (It does.)
3. Answers will vary; but they should include some way
of heating the air on one arm of the interferometer
without heating the air on the other arm or the interferometer itself.
21
Precision Interferometer 012-07137A
0
Experiment 3: The Index of Refraction of Glass
Reference to Procedure
1. The glass plate must be absolutely perpendicular to the
laser for accurate measurement of the index of refraction. When the plate is perpendicular, there will be a
faint secondary fringe pattern (Fabry-Perot interference
between the front and back surfaces of the plate) visible in the center of the view screen.
2. It is important to measure as large an angle as possible,
and measure the angle as carefully as possible.
Reference to Analysis
1. The actual equation, which is derived in Optics of the
Electromagnetic Spectrum, by C.L. Andrews
(Prentice-Hall, 1960) is
2
2
N
λ
(2t –Nλ
ng=
)(1 – cosθ)+(
0
2t(1 –cosθ) –Nλ
The second term is negligible for visible wavelengths,
and may be ignored.
4t
0
)
Notes General
It is often difficult to count large numbers of fringes due to
eyestrain. If you find this to be the case, you may want to
make a circuit such as this:
+5-15v
1k
10k
741
56k 20k
1k 1k
The phototransistor should be mounted in a plate of sheet
steel, which can then be held in the magnetic viewscreen
holder. Mask the transistor with a piece of electrical tape
with a pinhole at the center. Adjust the sensitivity of the
circuit with the 20k potentiometer so that the flashes of the
LED can be counted instead of the actual fringes.
+
This circuit may also be used in conjunction with the
PASCO Series 6500 computer interface so that the fringes
can be counted by computer, if desired.
22
012-07137A Precision Interferometer
Technical Support
Feedback
If you have any comments about the product or manual,
please let us know. If you have any suggestions on
alternate experiments or find a problem in the manual,
please tell us. PASCO appreciates any customer
feedback. Your input helps us evaluate and improve our
product.
To Reach PASCO
For technical support, call us at 1-800-772-8700
(toll-free within the U.S.) or (916) 786-3800.
fax: (916) 786-3292
e-mail: techsupp@pasco.com
web: www.pasco.com
Contacting Technical Support
Before you call the PASCO Technical Support staff, it
would be helpful to prepare the following information:
➤ If your problem is with the PASCO apparatus, note:
Title and model number (usually listed on the
label);
Approximate age of apparatus;
A detailed description of the problem/sequence of
events (in case you cant call PASCO right away, you
wont lose valuable data);
If possible, have the apparatus within reach when
calling to facilitate description of individual parts.
➤ If your problem relates to the instruction manual,
note:
Part number and revision (listed by month and year
on the front cover);
Have the manual at hand to discuss your
questions.
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