The criterion for destructive interference is very similar to the criterion for constructive interference. For destructive interference to occur, the difference between the two paths must be some integer number of wavelengths plus half a wavelength:

(m+12)λm,destructive=2d(m+12)λm,destructive=2d,
or
λm,destructive=2dm+12

What is the second-longest wavelength (λ1,destructive) that will not be visible, i.e., will have strong destructive interference for the reflected waves when reflected from a film of thickness 300 nm?

A) λ1,destructive = 450 nm.
B) λ1,destructive = 375 nm.
C) λ1,destructive = 225 nm.
D) λ1,destructive = 150 nm.