The Michelson Interferometer

(A Scratch program for simulating optics)

In this next example, there are 3 grey-blue rectangles, each representing a mirror. The little red balls are representing light beams. First, the red ball going in hits the first mirror (the one at an angle) and some of it is reflected and some of it passes through. The light that is reflected travels to the mirror at the top and the one that passed through is reflected by the third mirror (the one to the right). Both the balls then travel back to the first mirror and again some passes through and some is reflected, so there are two balls going downwards and two going to the left. This means that two red balls (or beams of light) join together in each direction. But by moving one of the mirrors, you can change at what point the balls meet. This is controlled by the ‘position’ slider and measured by the detector (at the bottom). This detector measures the time between the first ball hitting it and the second ball hitting it; this is recorded in the signal variable.

• At 100 on the slider, the mirror is furthest away and there is no difference between the balls, as they meet at the same time, so the signal says 0.
• At 0 on the slider, the top mirror is closest, so there is the biggest difference between the two balls meeting so the signal says 20.

This setup, in real life, would be called a Michelson Interferometer, which uses 3 mirrors to measure a distance. One mirror and the centre mirror are fixed while a second mirror can be moved. The signal then measures the motion of the second mirror.