Question

Two thin films of the same material but different thickness are separated by air. Monochromatic light is incident on the first film. When viewed normally from point A, the second film appears dark. From point B, on normal viewing:

(A) the first film will appear bright
(B) the first film will appear dark
(C) the second film will appear bright
(D) the second film will appear dark

Answer 1

The appearance of any film (that is, whether the film appears bright or dark) from any point in the plane depends on the interference pattern of the incident light in the film. If there is constructive interference of this light, then the films will appear bright and if there is destructive interference of this light, then the films will appear dark. We will proceed with the help of the data given to us in the problem.

Complete answer:
We will see the condition given in the problem. This is:
When viewed normally from point A, the second film appears dark. This means there has been a case of destructive interference at any of the films.
If we look at the second film from point B, it doesn’t really matter in which of the films destructive interference occurs. The second film from point B will still look dark, as for an observer at B, the films combined would appear to be producing a destructive interference.
So, the second film would appear dark from point B.
Hence, the second film will appear dark.

Hence, option (D) is the correct option.

Note:
It would be ambiguous to comment on the interference pattern formed on the first film from any of the two points. This is because there is no mention of interference patterns in the films individually. These questions are analytical in nature, so one should have the basic knowledge of the topic to solve problems like these.