Answer the questions.[Phy-5M]

Question

Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.

Vidyadip · Accepted Answer

Perform an experiment to demonstrate that light bends from its path, when it falls obliquely on the surface of a glass slab. Also show that angle of incidence is about equal to the emergent angle.

Take a glass slab and place it on a white sheet of paper fixed on a drawing board.
Mark the boundary ABCD of the glass slab.
Fix two pins P₁ and P₂, vertically on the drawing board such that line joining the pins is inclined to the edge AB of the glass slab (Figure).

Now, look through the glass slab from opposite side so that the images of pins P₁ and P₂ are seen exactly in line. Fix two pins P₃ and P₄, vertically on the drawing board such that pins P₃, P₄ and image of P₁ and P₂ are exactly in one line.
Remove the glass slab and all pins. Join the points P₁ and P₂ with a line and extend this line to touch the edge AB at P. Similarly join the points P₃ and P₄ with a line and extend the line to touch the edge DC at Q.
Join points P and Q with a straight line.
Draw normal NN’ at P on the edge AB and a normal N₁N₁' at Q on the edge DC.
Measure $\angle\text{P}_2\text{PN}$ and $\angle\text{P}_3\text{QN}_1'$ using a protractor or dee.

This experiment shows that when light falls obliquely on a glass slab, it bends along PQ from its original path along PL.

Also $\angle\text{P}_2\text{PN}\approx\angle\text{P}_3\text{QN}_1'.$ That is, angle of incidence is about equal to the emergent angle.

Note: Distance between pins P₁ and P₂, and between P₃ and P₄ must be large.

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