A beam of light of wavelength 650 nm is incident along the normal to two closely

Question

A beam of light of wavelength 650 nm is incident along the normal to two closely spaced parallel glass plates. For what air gap separation between the plates will the transmitted beam be of maximum intensity?

The part that confuses me is "will the transmitted beam be of maximum intensity?" Doesn't this mean that the problem has constructive interference? Also how many phase changes are there? Any detail on how the light moves through this scenario would be helpful and you don't have to do the calculations. I got the answer using 2nt= m, but I am just confused with how many phase changes and whether it is asking for constructive or destructive interference. Thanks

Explanation / Answer

Again, interference happens when light is reflected (not transmitted) from two sides of the air gap. So the problem is asking for destructive interference by "will the transmitted beam be of maximum intensity". As for phase change, it happens when light travels from media with smaller refractive index (n) to media with larger refractive index. Therefore, when the light travels from the first glass plate into the air gap, there is no phase change. And when the light travels from the air gap into the second glass plate, there is a phase change. Thus, you should use the equation for destructive interference with a phase shift (2nt=m?) to get the answer. Hope it's clear.

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