Question

In Young's experiment with white light, the central fringe is white. If now a transparent film is introduced in the upper beam coming from the top slit, then the white fringe :

• A. moves downward
• B. moves upward
• C. remains at the same place
• D. totally disappears

Answer 1

Answer: (B)

moves upward

Introducing a transparent film increases the distance traversed by the upper beam. As the distance traversed by upper beam increases, the lower beam has to travel a longer distance so as to have a constructive interference. The distance traveled by lower beam increases, if the white fringe is upper than the previous place.
Thus the white fringe moves upward.