For the purposes of [paraxial](Paraxial%20approximation.md) ray analysis a [mirrors](Mirrors.md) may be modeled as a lens. This is done with [ray transfer matrices](Ray%20transfer%20matrix.md) where we're aided by the fact that these matrices are independent of the beam direction (left or right) relative to the [ray optics](Optics%20(Index).md#ray%20optics). #Electromagnetism/Optics