Let q be the angle of incidence, and MD
Rationalised 2023-24
Physics
224
be the perpendicular from M on the principal axis Then,
ÐMCP = q and ÐMFP = 2q
Now,
tanq =
MD
CD  and tan 2q = 
MD
FD
(9 1)
For small q, which is true for paraxial rays, tanq  » q,
tan 2q » 2q Therefore, Eq