JEE Advanced 2020 · Paper 1 · Physics Q5

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PhysicsSingle correct+3 / -1Question 5 of 54

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)

Parallel rays of light travel left to right from air into a glass piece. The glass has a flat vertical left face; its right face bulges outward (convex) in the upper and lower parts and curves inward (concave) in the middle, meeting the flat top and bottom edges at the corners. Air on both sides.

A wavy vertical wavefront: its top and bottom ends and its middle lie to the right, and the two portions between them (upper and lower quarters) bow to the left.

A wavy vertical wavefront, the mirror image of (A): its top and bottom ends and its middle lie to the left, and the upper and lower quarters bow to the right.

A wavefront that is straight and vertical near the top and bottom, with a single bulge to the right in the middle.

A wavefront that is straight and vertical near the top and bottom, with a single bulge to the left in the middle.

Answer and worked solution

Official answer: A

Light is slower in glass, so wherever the glass is thicker the emerging wavefront lags (lies further to the left).

Thickness across the section, top to bottom.

At the top and bottom corners the glass is thin, so the wavefront is ahead (right).
Through the two convex bulges it is thickest, so the wavefront lags (left).
At the concave middle it is thin again, so the wavefront is ahead (right).

The wavefront is right-left-right-left-right from top to bottom, as drawn in (A).

Topics:Huygens' principle Refraction at plane surfaces

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JEE Advanced 2020 · Paper 1, Physics question 5