Thin Films: an example
Since we have preferentially reflected green light, the TOTAL phase difference must be 0 degrees (or 360 degrees).
Since we have nf > nw, we have 180 degrees due to reflection.
Therefore, we need 180 degrees due to extra distance, so need t = ?a/4nf where ?a = 500 nm, nf = 1.65, and so t = 500 nm / 4(1.65) = 76 nm.