72. For the first maximum m = 0 and for the tenth one m = 9. The separation is ∆y = (Dλ/d)∆m = 9Dλ/d.
We solve for the wavelength:
λ =
d∆y
9D
=
(0.15
× 10
−3
m)(18
m)
9(50
−2
= 6.0
−7
m = 600 nm .