Michelson Interferometer
Aims
Build a Michelson interferometer.
Observe interference fringes and understand why they are produced.
Perform various experiments using the interferometer.
Equipment
/ Laser Bench / / Laser Unit / / Lenses/ Optical Mounts / / Beam Splitter / / Mount
/ Adjustable Mirror / / Mirror
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.
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.
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.
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.
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…