JEE MAIN - Physics (2011 - No. 4)
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 - $$1$$ : When light reflects from the air-glass plate interface, the reflected wave suffers a phase change of $$\pi .$$
Statement - $$2$$ : The center of the interference pattern is dark.
Explanation
Statement - 1 : When light reflects from the air-glass plate interface, the reflected wave suffers a phase change of π.
This statement is true. According to the physics of wave optics, when light reflects from a denser medium (glass in this case) to a rarer medium (air), there is a phase change of π (or 180 degrees).
Statement - 2 : The center of the interference pattern is dark.
This statement is also true. When a thin air film is formed between a convex lens and a plane surface, the path difference at the center of the pattern is zero. However, due to the phase change of π at the lower surface (glass surface), the two interfering waves are out of phase at the center, resulting in destructive interference and creating a dark spot at the center.
So both statements are true. However, statement - 2 is not a direct explanation of statement - 1. Statement - 2 arises because of the combination of the path difference and the phase change mentioned in statement - 1, but it doesn't directly explain why the phase change occurs.
Hence, the answer is :
Option B : Statement - 1 is true, Statement - 2 is true, Statement - 2 is not the correct explanation of Statement - 1.
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