Are there interference effects for light that is reflected at an angle?

Question

I am imagining a glass substrate with a thin coating that has a 1/4th wave optical thickness. I understand how this acts as a basic anti-reflective coating for the one wavelength for which it acts as a quarter wave. The light reflected from the coating and the substrate cancel each other destructively.

Now I am thinking about light coming in at an angle. I was thinking that the light that came in at an angle would have to take a longer optical path length so that it would act like a quarter wave for a longer wavelength. However I modeled this in OpenFilters and I saw that the opposite was true. Light incident at an angle seemed to be acting like a quarter wave for a shorter wavelength. I was wondering if anyone has a explanation or a good resource to read about the reflection of incident light at an angle and how interference effects work in that case.

Answer 1

OpenFilters is wrong (or your programming or interpretation of it is). Interference depends on the difference in path lengths between substrate and coating, and that increases with shallower angles of incidence.

Don't use OpenFilters, use a puddle.

Oil your finger, wipe off almost all the oil, and stick it into the puddle. You will create a thin layer of oil on the surface - practically a monolayer, if you do it right. You can then look at the puddle from different angles and see exactly how the apparent colour changes because of interference. In general you will get the brightest colours with the steepest angle of incidence, because the shallower, even grazing, angles will have a path length difference of much more than 300-700nm.