For a small angle prism, i e , a thin prism, Dm  is also very small, and
we get
(
)
21
/2
sin[(
)/2]
sin[
/2]
/2
m
m
A
D
A
D
n
A
A
+
+
=
≃
Dm = (n21–1)A
It implies that, thin prisms do not deviate light much 9