When white light is incident normally from air (n=1.0) onto a soap film in air (n=1.33), two reflected rays—one from the top surface and one from the bottom surface of the film—interfere. Which of the following correctly describes the phase change(s) that occur upon reflection?
a) Neither reflected ray undergoes a phase shift.
b) Both reflected rays undergo a 180∘ phase shift.
c) Only the ray reflected at the top surface undergoes a 180∘ phase shift.
d) Only the ray reflected at the bottom surface undergoes a 180∘ phase shift.