Answer
a) bright
b) I
Work Step by Step
(a) We know that as the thickness of the film decreases, the phase difference between the rays goes to zero and satisfies the condition for constructive interference. As a result, the film appears bright.
(b) We know that the best explanation is option (I) -- that is, the film will appear bright because as the thickness of the film goes to zero, the phase difference for the reflected rays goes to zero.
