There is a soap film of thickness t in air. I don't understand intuitively why a film thickness of of t = lambda/4 where lambda is the length of the light that is incident upon the film, results in maximum brightness of the light. I know that this is true from the equation for interference but I don't understand the equation. Also, here is another inference problem: If two wave sources have different wavelengths, what do the nodal and antinodal lines look like? I am thinking that there will still be interference, but the lines will shift. Will they still be lines?