A parallel beam of light strikes a piece of transparent glass having cross section as shown in the figure below. Correct shape of the emergent wavefront will be (figures are schematic and not drawn to scale)-
(A)(see structure)✓ Correct
(B)(see structure)
(C)(see structure)
(D)(see structure)
Answer: (A)
Solution
When a parallel beam (plane wavefront) enters the glass piece with varying thickness, different parts of the wavefront travel through different thicknesses of glass. The thicker portions cause greater optical path delay. Since the glass is thicker in the middle and thinner at the edges, the central portion of the wavefront is delayed more than the edges. Upon emergence, this creates a wavefront that is concave (curved inward) as shown in option (A).
JEE Advanced · 2020 · 27 Sep 2020 · Paper 1 · Single correct option
