Biprism, Thin Films & Other Devices
This question has a paragraph followed by two statements, Statement and Statement . 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 plane. 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 - : When light reflects from the air-glass plate interface, the reflected wave suffers a phase change of
Statement - : The center of the interference pattern is dark.
Show answer and solution
Answer: (b)
- Statement 1: reflection at air to glass (rarer to denser) gives a phase change of . True.
- Statement 2: at the centre the film thickness is zero, and the extra makes the two reflected waves cancel, so the centre is dark. True.
- Statement 1 explains Statement 2, not the other way round, so Statement 2 is not the explanation of Statement 1.
- So the answer is (b).