Question

What is the function of the mirror and the water?

Answer 1

Answer:

Water is a dielectric material. At normal incidence it reflects about 4% or less of the incident light. This is why if you are standing directly above a body of water, you can look down and see a reflection but also see the bottom of the pond or lake, at least in the shallow parts.

If you look at higher angles of incidence, the amount of reflection increases. (We will ignore polarization and Brewster’s angle for the moment.)

Since the bottom of the body of water is often dark, all that you see looking across the body of water is the reflection, which can be nearly 100% for grazing incidence.

If the water is choppy, then in a lot of places you won’t have grazing incidence, and in those places you will have much reduced reflection. The random angles also tend to ruin the virtual image in the reflection.

But if the water is still, all of the surface is near grazing incidence and you see a mirror-like reflection.

It is true that the total reflected light increases with angle of incidence, but the reflectance for the two primary linear polarization states is dramatically different. In fact, one state, the vertical polarization, actually goes through a minimum of zero. This is why you can wear polarized sunglasses and filter out a lot of the reflection. The angle of incidence where this minimum occurs is called Brewster’s angle and is very useful for polarization studies. However, near grazing incidence, both polarization states are highly reflected and polarized sunglasses are ineffective for very still water when viewed at grazing incidence, leaving a totally mirror-like reflection.