An interferometer is a device that was first used by Michelson and Morley in their famous experiment. These days it is usually used in field of optics.

It can be used to measure the wavelength of a light source, to detect minute changes in distance, as an extremely high precision ruler as well as a number of other uses.

Design of Apparatus

In the centre of the cross is a mirror known as a beam splitter. This mirror is only 50% reflective. This means that half the light that hits it passes right through it the other half is reflected as usual.

How it works

We assume that the light source is placed at the end of arm1. The beam splitter is then placed at 45^o to the incoming light beam, also known as the incident light.

One of the laws of reflection states that the angle at which incoming light hits a mirror is equal to the angle at which it is reflected. In other words. We draw an imaginary line perpendicular to the mirror at the point where the incoming light strikes. We then measure the angle between this perpendicular line and the incoming light ray, this is known as the angle of incidence. The angle at which the light ray is reflected is exactly equal to the angle of incidence.

This means that half the light is reflected through 90 degrees along arm2, beam2. The rest is transmitted through the mirror and down arm3, beam3. The distance form the beam splitter to each of the mirrors M2 and M3 is exactly the same, usually to a precision of nanometres³ or more. After the light is reflected by the mirrors it passes back down the same way it came.

When the light reaches the beam splitter again half is reflected and half is transmitted. This means half of beam2 is reflected back down arm1 and half passes through into arm4, and vice versa for beam3. We ignore the light going down arm1.

Now at the end of arm4 there will be either a screen or a lens that focuses the light onto a screen.

Congratulations. We have just succesfully split a light beam and recombined it again, who's to say we did anything to it at all!

Use of the Interferometer

This means the distance that beam2 travels can be changed. A millionth of a metre may not sound like that much of a difference, but it is perfect for what we are about to do.

We are assuming that light is travelling as a wave. This tiny difference in the distance that beam2 travels means that a diffent part of the wave hits the mirror than did previously.

Before both beam2 and beam3 travelled the exact same distance so we can assume that both light waves were at a crest, the top of the wave, when they hit the mirrors M2 and M3, respectively.

If M2 were moved a distance equal to half the wavelength of light then when beam2 hits the mirror it will be at a trough, the bottom of the wave. This means that when the two beams recombine at the screen at the end of arm4 they will cancel each other out leaving a dark patch.

In reality we cannot make the light do this exactly because there are in fact millions of different waves making up the beam of light, but they are related to each other so what you do get is a pattern of light and dark fringes, as they are known. This pattern is called an interference pattern, because the two waves are interfering with each other.

For this to work a coherent light source needs to be used. A coherent light source is one from which all the light leaving it is of the same frequency, i.e. a laser.

Uses of the interferometer.

In this way if a light of unknown wavelength is shone onto the beam splitter, careful adjustment of M2 will allow us to calculate the wavelength and therefore the frequency of the light⁴.

If M2 is connected by some means to a system in which a tiny movement is occuring this will show up on the screen as the interference pattern changing. This change allows us to work out the size of the movement in the other system.