scientific
Interferometer
012-02675B
2
Theory of Operation
LASER
VIEWING SCREEN
M2
M1 (FIXED MIRROR)
BEAM-
SPLITTER
(MOVABLE
MIRROR)
Figure 1 MICHELSON INTERFEROMETER
Interference Theory
A beam of light can be modeled as a wave of oscillat-
ing 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 mini-
mum 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 correla-
tion between the frequency and phase of the oscilla-
tions 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.
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.
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 hypoth-
esized 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 interferom-
eter. 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
1
, the other is
transmitted toward mirror M
2
. M
1
and M
2
reflect the
beams back toward the beam-splitter. Half the light
from M
1
is transmitted through the beam-splitter to the
viewing screen and half the light from M
2
is reflected
by the beam-splitter to the viewing screen.
