PASCO OS-9257A User Manual

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

PleaseFeel 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: [phone]
FAX: [phone]
email: [e-mail]
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,
Michelsons 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, Youngs experiment offered important
evidence for the wave nature of light.

Youngs 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