Help please - waves question

A coating of film n=1.37 on glass slabs (n=1.6) is 8.45x10^-5 cm thick. If white light is incident normally, which visible wavelengths are missing in the reflected light?

White light containing wavelengths from 400nm to 700nm.

Answer in order of descending wavelength.

Question

Help please - waves question

A coating of film n=1.37 on glass slabs (n=1.6) is 8.45x10^-5 cm thick. If white light is incident normally, which visible wavelengths are missing in the reflected light?
 
White light containing wavelengths from 400nm to 700nm.

Answer in order of descending wavelength.

anonymous · Answer

Light will be reflected from the first interface (air-film) and the second (film-glass).  Any wavelength for which these two reflected waves exactly cancel will be missing.