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.
(Show it mathematically or write it in 8 points)
(LAQ - 4 Marks)
Answers
A ray of light, incident obliquely on a face of a rectangular glass slab placed in air, emerges from the opposite face parallel to the incident ray. Draw labelled diagram to show it. State two factors on which the lateral displacement of the emergent ray depends.
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.
Answer:
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 P
1
and P
2
, vertically on the drawing board such that line joining the pins is inclined to the edge AB of the glass slab (Refer Figure).
Now, look through the glass slab from opposite side so that the images of pins P
1
and P
2
are seen exactly in line. Fix two pins P
3
and P
4
, vertically on the drawing board such that pins P
3
,P
4
and image of P
1
and P
2
are exactly in one line.
Remove the glass slab and all pins. Join the points P
1
and P
2
with a line and extend this line to touch the edge AB at P. Similarly join the points P
3
and P
4
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 N
1
N
1
′
at Q on the edge DC.
Measure ∠P
2
PN and ∠P
3
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 ∠P
2
PN≈P
3
QN
1
′
. That is, angle of incidence is about equal to the emergent angle.
Note : Distance between pins P
1
and P
2
, and between P
3
and P
4
must be large