Direction:
The question has a paragraph followed by two statements, Statement -1 and statement -2 . Of the given four alternatives after the statements, choose the one that describes the statements.
A thin air film is formed by putting the convex surface of a plane - convex lens over a plane glass plate. With monochromatic light, this film gives an interference pattern due to light reflected from the top (convex) surface and the bottom (glass plate) surface of the film.
Statement-1 : When light reflects from the air-glass plate interface, the reflected wave suffers a phase change of .
Statement-2 : The centre of the interference pattern is dark.
The question has a paragraph followed by two statements, Statement -1 and statement -2 . Of the given four alternatives after the statements, choose the one that describes the statements.
A thin air film is formed by putting the convex surface of a plane - convex lens over a plane glass plate. With monochromatic light, this film gives an interference pattern due to light reflected from the top (convex) surface and the bottom (glass plate) surface of the film.
Statement-1 : When light reflects from the air-glass plate interface, the reflected wave suffers a phase change of .
Statement-2 : The centre of the interference pattern is dark.
- (A)Statement-1 is true, Statement-2 is true; Statement-2 is the correct explanation of Statement-1.✓ Correct
- (B)Statement-1 is true, Statement-2 is true; Statement-2 is not the correct explanation of Statement-1.
- (C)Statement-1 is false, Statement-2 is true.
- (D)Statement-1 is true, Statement-2 is false.
Answer: (A)
Solution
Both statements are true. When light travelling in the thin air film reflects at the lower air-to-glass boundary it passes from a rarer to a denser medium, and the reflected wave picks up a phase change of . At the point of contact the film thickness is essentially zero, so the geometric path difference is negligible, yet that extra makes the two reflected beams interfere destructively, which is why the centre of the pattern comes out dark. The dark centre is thus a direct consequence of the phase change on reflection, so both statements are true and the second stands as the correct explanation of the first.
JEE Main · 2011 · 10 Apr 2011 · Offline · Single correct option