Practice portal › Wave Optics › Interference and Young's Double Slit Experiment

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 π.
Statement-2: The centre of the interference pattern is dark.

Asked in JEE Main 2011 · Slabs, media and modified set-ups

• (1) Statement-1 is true, Statement-2 is false.
• (2) Statement-1 is true, Statement-2 is true, Statement-2 is the correct explanation of Statement-1.
• (3) Statement-1 is true, Statement-2 is true, Statement-2 is not the correct explanation of Statement-1.
• (4) Statement-1 is false, Statement-2 is true.

Step-by-step solution

Why the other options are wrong

• (1) The centre of Newton's rings in reflected light is indeed dark, so Statement-2 is true.
• (2) The dark centre is a result of the phase change, not its cause, so Statement-2 cannot explain Statement-1.
• (4) Reflection at a denser medium (air to glass) does give a phase change of π, so Statement-1 is true.

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