What's the actually meaning of phase change?

[KFC]

In fundamental optics, we know that "There is no phase change when the wave is reflected from a boundary leading to a medium of lower index of refraction"

So, does it mean if a light is incident on a 100% reflected mirror, it will induce a phase change? If there is a phase change, why we doesn't feel it by looking at the image?

And by the way, I saw something on the paper which is like a plate, place at 45 degree, the light coming from above will pass through the plate and the light coming form the left will be completely reflected (since the light is making 45 degree with the plate, the outgoing light is perpendicular to the incoming light). What's the name of this plate? Will it induce a phase change for the left-incoming light?

 

[Almsco]

No, there will be no phase change when the wave is reflected from a boundary leading to a medium of lower index of refraction. The mirror you are referring to would be considered a perfectly reflecting surface, and thus there would be no phase change.

The plate you are referring to is called a half-wave plate. It is designed to induce a phase change in the light that passes through it, depending on the orientation of the plate. When the plate is placed at a 45 degree angle, the light coming from above will pass through the plate and the light coming from the left will be completely reflected. The left-incoming light will experience a phase change when it is reflected off of the plate.