f is (approximately) half the radius of
curvature R f is negative  for concave mirror; f  is positive for a convex
mirror 5 For a prism of the angle A, of refractive index n 2 placed in a medium
of refractive index n1,
n
n
n
A
D
A
m
21
2
1
2
2
=
=
+
(
)


(
)
sin
/
sin
/
where Dm is the angle of minimum deviation