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.