I was discussing the reason why we see beating from a Michelson interferometer, and one of my friend said it 's because the light have different frequencies, therefore, they would be out of phase.
But my argument is, Because two frequencies travel at the same speed, their phase difference remain the same. But when you keep adjusting the movable mirror of Michelson interferometer, you can shift their phase difference more quickly, and that's why you get beating phenomena.
What is the actual source of beating?