Question

In the Young's experiment with white light, the central fringe is given as white. Now a transparent film is introduced in the upper beam which is coming from the top slit, then the white fringe will be given as,
A. moves downward
B. moves upward
C. remains at the same place
D. totally disappears

Answer 1

Hint: The introduction of a transparent film can make changes in the velocity as well as the direction of the beam. When a transparent film is introduced, the distance in order to traverse will increase. The fringe produced at this condition is to be white when there should be a constructive interference taking place. This all will help you in answering this question.

Complete step by step answer:
When we introduce a transparent film in the path of the beam, it will increase the distance traversed by the upper beam. As the distance traversed by the upper beam increases, then the lower beam will need to travel a longer distance. This is in order to create a constructive interference. If the white fringe is fixed in an upper region than the previous place. Then the distance travelled by the lower beam increases. Therefore the white fringe will be moving upward. Therefore we have arrived at the conclusion that the fringe will be moving upwards when a transparent plate is placed in the upper beam. This has been mentioned as option A.

Note:
When a light wave is passing from a rarer medium to a denser medium, then there will be a change in phase of $\pi $ radians. Thus the difference will give a variation in the situations for constructive and destructive interference. This phenomenon will be the reason for the formation of interference patterns in thin film like oily layer, soap film and so on.