Physics

The key is tracking how the wavelength changes as light moves between media with different refractive indices, then counting how many wavelengths fit into each layer—this determines the total phase accumulated. When light travels through a medium with refractive index $n$, its wavelength shrinks to $\lambda/n$, so a thickness $d$ contains $nd/\lambda$ wavelengths. Sum these across all three layers to find the total optical path length, which tells you whether the beam emerges in phase or out of phase with itself.

The key is tracking how the wavelength changes as light moves between media with different refractive indices, then counting how many wavelengths fit into each layer—this determines the total phase accumulated. When light travels through a medium with refractive index $n$, its wavelength shrinks to $\lambda/n$, so a thickness $d$ contains $nd/\lambda$ wavelengths. Sum these across all three layers to find the total optical path length, which tells you whether the beam emerges in phase or out of phase with itself.