78.
(a) Using the notation of Sample Problem 37-6,
L =
D
1.22λ/d
=
2(50
× 10
−6
m)(1.5
−3
m)
1.22(650
−9
= 0.19 m .
(b) The wavelength of the blue light is shorter so L
max
∝ λ
−1
will be larger.