For the derivation of the focal length of a planoconvex lens with one surface silvered, we consider the phenomenon taking place in three steps:
- Refraction through first surface
- Reflection through silvered surface
- Refraction through first surface again
So the inverse of the resultant focal length can be obtained as the summation of twice the inverse of the focal length of the planoconvex lens and the inverse of the focal length of the silvered surface.
My question here is:
Why do we consider the focal length of the planoconvex lens (while taking into consideration the refracting surface) and why not just the focal length of that particular refracting surface?