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If we shine a laser with λ = 600 nm on a thin film of a material with refractive index 1.5, at what thicknesses will we get constructive/destructive interference?

The transmitted light should have a λ of 400 nm. So if the film is 400 nm thick, the light will complete exactly one oscillation before bouncing back, completing another one INSIDE the film - and then rejoining for maximum constructive interference.

But just from doodling oscillations, it seems as if it will reach maximum constructive interference if the film is 200 nm thick aswell. And maximum destructive interference at 100 nm and 300 nm... Is this correct? The mathematical derivation was very confusing. When does max interference occur?