A retroreflector is a device composed of either a system of [[Mirrors]] or a single spherical structure that reflects back a light ray towards its source. Of course, it is possible to use a simple [planar mirror](Mirrors.md#Planar%20mirrors) as a retroreflector. However, this isn't practical because a planar mirror only reflects a beam back towards its source if it is exactly perpendicular to the [optical axis](Optics%20(Index).md#ray%20optics). # Corner reflector # Ppherical reflector A [ball lens](Ball%20lens.md) may be used as the basis for a retroreflector under the condition that the entire interior back half of the ball lens is composed of a reflective coating. The condition under which a ball lens acts as a retroreflector is if the [Index of refraction](Index%20of%20refraction.md) inside the lens is twice that of the index of refraction outside of the lens (i.e. $n_2=2n_1$). Why is that? consider the relationship between the [elements](Ray%20transfer%20matrix.md#The%20matrix%20elements) of a ray transfer matrix and the in an optical system. consider that the back focal length (as shown in the diagram [here](Ray%20transfer%20matrix.md#The%20matrix%20elements)) is $-A/C$ where $A$ and $C$ are ray transfer matrix elements. For the optical element to act as a retroreflector, $-A/C=0.$ If we look at the [ball lens ABCD matrix](Ball%20lens.md#ABCD%20matrix) we find that for $-A/C=\frac{R}{2}\frac{2-n_2/n_1}{n_2/n_1-1}=0$, $n_2=2n_1.$ #Electromagnetism/Optics #ExperimentalTools/ExperimentalToolsinOptics