Question

What is the additional phase difference introduced when light is reflected from a rarer medium?

Answer 1

When the ray of the light's phase changes by 180° when the reflection is on a medium which has greater Refractive index in comparison to the medium in which it is traveling. A light wave traveling through the air is reflected by a glass which may make difference of 180° phase change.

Answer 2

The phase difference between a node and its nearest antinode is π2π2 or 90 degrees. This can be seen by thinking of the wave as a simple sine function. There is a node at 0, then again at ππ, before the whole thing begins to repeat at 2π2π. The antinodes are half way between each pair of adjacent nodes, at π2π2, 3π23π2, etc.