# A Question About Thin Film Interference

I learn in high school Physics class that when light travels from a medium to a more optically dense thin film, then the reflected rays will destructively interfere with each other. My questions are:

1. Will the interference of the rays being reflected off the top surface with the rays being reflected off the bottom of the film ever be constructive? Can you explain why it will be constructive?

2. Then in what situations will the rays destructively interfere with each other?

3. And if the reflected ray (the one reflected off the bottom of the thin-film) can be made so that it constructively interferes, how can it then be said that the bottom wave is always 180 degrees out of phase? Because to constructively interfere, you cannot be 180 degrees out of phase? So how can it be said that all reflected rays of the bottom of a more optically dense material will be 180 degrees out of phase with the reflected ray on top?

Thank you so much! This would immensely help my understanding of the topic! Please, though, try to answer the question at the level of a high school student.

• Could you please consider writing a more descriptive question title? See this meta post: How do we write good question titles?.
– user191954
Oct 23, 2018 at 14:15

In addition, if the refractive index of a medium the light is coming from is smaller that the refractive index of a medium the light is going to, the phase of the reflected light will be flipped by $$180^{\circ}$$.
As an example, if, for a given thin film and angle of incidence, the difference in path lengths is equal to one wavelength and the reflections from both top and bottom surfaces undergoes $$180^{\circ}$$ phase shift, the interference will be constructive. If only one of the reflections underwent $$180^{\circ}$$ phase shift, the interference would be destructive.