Question

Why is there a 90 degree phase change when light reflects off a mirror, for example in lloyds mirror?

Answer 1

180 degrees i mean

Answer 2

because just like waves on a string, when a wave moves from a lower index of refraction to a higher one, your phase will flip. Again, you can check by making a wave with a string against a wall. However, when moving from lower higher index to lower index, it doesn't. Light is electromagnetic wave, which also follows the wave equation

Answer 3

When light from the rarer medium to the denser medium gets reflected from the denser medium then it introduces a phase change of pi radians in the reflected light.