Consider a film of thickness L as shown in four different cases belew. Notice the observation of film with perpendicularly falling light. Mark the correct statememt(S).

Options

A. For (1) and (2), the reflection at film interfaces causes zero phase difference for two reflected rays.
B. For (2) and (3), the reflection at film interfaces causes a phase difference of `pi` for two reflected rays.
C. For (1), the film will appear dark, if it is observed through reflected rays from film interfaces.
D. For (3), the film will appear dark, if it is observed through reflected rays from film interfaces.

Answer

b.,d

Explanation

It is better to make a chart for all the four cases which show the phase difference due to reflection at top and bottom surfaces of film.
`{:(,underset("surfaces")("Top"),underset("surfaces")("Bottom"),"Due 2L","Total"),("Case I",0,0,6 pi,6 pi),("Case II",0,pi,6 pi,7 pi),("Case III",0,pi,6 pi,7 pi),("Case IV",pi,pi,6 pi,8 pi):}`.