Page 80 - Physics - XII

EXPERIMENT - 5

Aim

To determine angle of minimum deviation for a given prism by plotting a graph between angle of incidence
and angle of deviation.

Apparatus and Materials Required
A drawing board, a triangular glass prism, a white sheet of paper, allpins/drawing pins, a pencil, a scale, a
graph paper, and a protractor

Principle/Theory A
Consider the principal section ABC of the prism. A ray of light
PQ (from air to glass) incident on the surface AB at an angle i is Q M D
refracted at an angle r along QR and fi nally emerges out along RS i r r′ e
as shown in the Fig. 5.1. P R
The dotted lines in the fi gure represents the normal to the surfaces. S
The angle of incidence at the second surface AC (from glass to air)

is r' and the angle of refraction or emergence be e. The angle of B C
deviation D is between the direction of incident ray PQ produced Fig. 5.1: Refraction of light through
forward and the direction of emergent ray RS produced backward. a prism
From geometrical considerations, we have
A = r + r'
and D = ∠MQR + ∠MRQ = (i – r) + (e – r')
D = i + e – A …(i)

For minimum deviation D , the light ray passes through the prism symmetrically, i.e. parallel to the base,
m
so that
D = D , i = e ⇒ r = r'
m
At the minimum deviation of position of the prism, we get the brightest image of an object.

Procedure

T
1 N 1 T N N N
P 1 2 2 T 3 3 T 4

30° P 2 P 3 4 P 4
Q 35° 40°
1 O Q 2 O Q O Q 45° O
X B 1 A B 2 F A B 3 3 F A B 4 4 A X′
F
1 2 3 F 4
D 1 D 4 E
D D 4
R R 2 R 3 R 4
C 1 C 2 C 3 E 3 C S
S S S 4
1 E 1 2 E 2 3

Fig. 5.2: Refraction through prism at diff erent angles of incidence

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