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?