PASCO OS-8501 User Manual

Instruction Manual and
Experiment Guide for
the PASCO scientific
Model OS-8501

Interferometer

012-02675

10/91

Revision B

MODEL OS-8501 INTERFEROMETER

scientific

Copyright © February 1986 $10.00

Interferometer 012-02675B

Table of Contents

Section Page

Copyright, Warranty, and Equipment Return........................................ ii

Introduction...............................................................................................1

Equipment .................................................................................................1

Equipment included:
Additional Equipment Needed:
Additional Equipment Recommended:

Theory of Operation .................................................................................2

Interference Theory
The Michelson Interferometer

Operation

The Interferometer.........................................................................................4

The Movable Mirror .......................................................................................4

Aligning the Interferometer ..........................................................................5

Exp 1: Measuring the Wavelength of Light .......................................... 7

Exp 2: Measuring the Index of Refraction for Air ................................ 8

Appendix

Maintenance.................................................................................................11

Replacement Parts ......................................................................................11

Inteferometry with a Spectral Light Source ..............................................12

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012-02675B Interferometer

Copyright, Warranty and Equipment Return

Please—Feel free to duplicate this manual
subject to the copyright restrictions below.

Equipment Return

Copyright Notice

The PASCO scientific Model OS-8501 Interferometer
manual is copyrighted and all rights reserved. How­ever, permission is granted to non-profit educational
institutions for reproduction of any part of this 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.

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.

Limited Warranty

PASCO scientific warrants this 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. This 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.

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:

➀ The packing carton must be strong enough for the

item shipped.

➁ Make certain there are at least two inches of

packing material between any point on the
apparatus and the inside walls of the carton.

➂ 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

scientific

Phone: [phone]
FAX: [phone]
email: [e-mail]
web: www.pasco.com

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012-02675B Interferometer

Introduction

The PASCO scientific Model OS-8501
Michelson Interferometer is a precision instru­ment capable of measuring the wavelength of
visible, monochromatic light with an
accuracy of better than 5%. With the
included vacuum chamber, it can also be
used for precise measurements of the index of
air refraction.

➧ CAUTION: Avoid touching all optical
surfaces on the interferometer, because
minute scratches can impair the clarity of
the interference image. For instructions
on cleaning the optical surfaces, see the
Maintenance section at the end of this
manual.

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MODEL OS-8501 INTERFEROMETER

Equipment

Equipment included:

• Interferometer base with built-in micrometer
and leveling feet

• Movable mirror

• Beam splitter

• Three point adjustable fixed mirror

• Vacuum cell for measuring the index of
air refraction

• Beam expanding lens with component holder

• Fitted case

Additional Equipment Needed:
Light source: To operate the Michelson Interferom-

eter you will also need a monochromatic light source,
preferably a laser. We recommend the PASCO 0.5
mW He-Ne Laser (Model OS-9171), but any low
power laser that operates in the visible range will
work. For optimum ease of alignment, the level of the
beam should be 1.5 inches (3.8 cm) above the bench
top. Leveling screws on the interferometer allow the
height to be adjusted.

Vacuum pump: To measure the index of refraction of
air, you will also need a vacuum pump. The PASCO
Hand Vacuum Pump (Model OS-8502) is an accurate
yet relatively inexpensive pump with a built-in gauge.
It allows precise control of the vacuum level when
counting fringes. However, the vacuum chamber can
be used with any pump that can be connected by a 1/4
inch (0.64 cm) I.D. (inner diameter) tube.

➧ CAUTION: Do not use the vacuum chamber
with a compressor; it is not built to withstand
positive pressures.

Additional Equipment Recommended:

The PASCO Optics Bench can function as an aid in
aligning the interferometer. It simplifies the alignment
procedure and the magnetic pads on the bench top hold
the laser and interferometer firmly in position once the
system is aligned. A 1.0 m Optics Bench can be pur­chased separately (Model OS-9103). A 70 cm optics
bench is included as an integral part of the PASCO
scientific Introductory Optics System (Model OS-

8500).

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Interferometer 012-02675B

LASER

VIEWING SCREEN

M

2

M1 (FIXED MIRROR)

BEAM-

SPLITTER

(MOVABLE

MIRROR)

Theory of Operation

Interference Theory

A beam of light can be modeled as a wave of oscillat­ing electric and magnetic fields. When two beams of
light meet in space, these fields add according to the
principle of superposition. At each point in space, the
electric and magnetic fields are determined as the
vector sum of the fields of the separate beams.

If the two beams of light originate from separate
sources, there is generally no fixed relationship
between the electromagnetic oscillations in the beams.
If two such light beams meet, 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 much 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.

However, if the two beams of light originate from the
same source, there is generally some degree of correla­tion between the frequency and phase of the oscilla­tions of the two beams. 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 two beams may be continually out of
phase and a minima, or dark spot, will be seen.

The Michelson Interferometer

In 1881, some 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
method to test for the existence of the ether, a hypoth­esized medium in which light could propagate. Due in
part to his efforts, the ether is no longer considered a
viable hypothesis. Michelson’s interferometer has
become a widely used instrument for measuring the
wavelength of light, and for using the wavelength of a
known light source to measure extremely small
distances.

Figure 1 shows a diagram of a Michelson interferom­eter. A beam of light from the laser source strikes the
beam-splitter. The beam-splitter is designed to reflect
50% of the incident light and transmit the other 50%.
The incident beam therefore splits into two beams; one
beam is reflected toward mirror M
transmitted toward mirror M
beams back toward the beam-splitter. Half the light
from M1 is transmitted through the beam-splitter to the
viewing screen and half the light from M

by the beam-splitter to the viewing screen.

, the other is

1

. M1 and M2 reflect the

2

is reflected

2

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 became visible. When first performed, Young’s
experiment offered important evidence for the wave
nature of light.

Young’s slits function 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.

Figure 1 MICHELSON INTERFEROMETER

2

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