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Michelson Interferometer
Aims
Build a Michelson interferometer.
Observe interference fringes and understand why they are produced.
Perform various experiments using the interferometer.
Equipment
Laser Unit
Lenses
Optical
Mounts
Beam Splitter
Mount
Adjustable
Mirror
Mirror
Laser
Bench
A soap bubble and an oil slick are common examples of interference between light
waves. Some colours interfere in a destructive manner and disappear; other colours
constructively interfere and appear brighter Figure 1
Figure 1. Interference of two input waves
The Michelson Interferometer has many applications from measuring light
wavelengths and the quality of optical surfaces such as the ends of optical fibres, to
visualising engineering phenomena and detecting gravity waves.
Observe the laser safety rules. Use the screen or a piece of card to
locate the laser beam. View from above only.
Activity
Plug the laser into the upper socket on the laser bench and set up the components on
the laser bench by reference to figure 2
The beam splitter is a semi transparent silvered piece of glass that both reflects and
transmits light. The beam splitter needs to be set at 45 degrees to the laser and sits in a
special base. Do not touch the surface!!
The mirrors should be about the same distance away from the beam splitter and can
be lightly clamped with the plastic screws.
Screen
beam
splitter
adjust
mirror
fixed
mirror
Figure 2 The Michelson interferometer
To align the interferometer, align the two laser spots so that they are on top of each
other. Once the spots are aligned, defocus the laser so that the beam is about 4cm
diameter at the screen.
Final adjustment is through the screws on the back of the adjustable mirror. The spots
should flicker or light and dark bands appear to show interference is happening. This
may take a few minutes of patience because the interferometer is a high precision
instrument.
Small spots on the laser lens may cause some distortion, but you will recognise the
interference pattern by its sensitivity to very small movements. Once you have
aligned the mirrors and see the interference pattern sketch what you see in the box
below.
Now try to get a circular fringe pattern. This can only be observed when the mirrors
are almost exactly the same distance from the screen.
Sketch the pattern.
wavefronts
mirror
Screen
Figure 3 Explanation of a Circular fringe pattern.
The circular pattern occurs when the mirrors are perfectly aligned. As the light comes
from a small source the rays travel out at an angle, so travel slightly different
distances to and from the mirrors.
The interferometer can be used to measure optical surfaces or structures. Keep the
circular fringe pattern and insert the lenses in one arm of the interferometer.
Screen
beam
splitter
lens
adjust
mirror
fixed
mirror
Figure 4 Inserting the lenses in the arm of the interferometer
Record the patterns for three different lenses.
Explain how the lenses might change the interference pattern given the speed of light
in air and these media is different.
Tap the laser bench or table with your hand. Note how sensitive the pattern is to
vibrations. This isn’t surprising as the interference pattern is generated from light
waves 650nm or 0.000650mm apart. A movement of a quarter of a wavelength can
turn a dark band into a bright band. This sensitivity means it is an ideal instrument for
measuring small seismic vibrations or for predicting volcano eruption.
You may also wish to try putting a candle in one of the arms of the interferometer.
Screen
beam
splitter
adjust
mirror
candle
fixed
mirror
What do you observe? Can you think of a reason why you see this?
Record your observations
You may wish to research the detection of gravity waves and how a device like this
may be used to see the hidden universe…