Two thin films of the same material but different thickness are separated by air. Monochormatic light is incident on the first film. When viewed normally form point A, the second film appears dark.

Options

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

d

Explanation

Path difference between waves of
`I` is `rarr 2 mu t_(1)`
`II` is `rarr 2 mu t_(2)`
`III` is `rarr 2 mu t_(3)`
System `I` is used to observe film 1 and A, system II is used to observe film 2 form B, and system III is is used to observe film 2 from A. It is given that `2 mu t_(2) =` odd multiple of `lambda//2`, i.e., dark. So, from A film 2 appears dark while for I, relation between `t_(1)` and `t_(2)` has to be known.