**The Refraction through a Prism Viva Questions**

*(i) What is meant by refractive index of the material of a prism?*
Ans: It is the ratio of the velocity of light in free space
to the velocity of light in the material

*(ii) What is the relation between angle of incidence and angle of emergence, when the angle of deviation is minimum?*
Ans: The angle of incidence and angle of emergence are
equal. The ray passes symmetrically through the prism.

*(iii) Does the angle of minimum deviation depends on the colour of light?*
Ans: Yes

*(iv) What is critical angle of a medium?*
Ans: It is the angle of incidence in the medium for which
angle of refraction in air is 90°

*(v) What is the relation between refractive index (n) and critical angle C?*
Ans: n = 1/ sin C.

*(vi) What is the unit of refractive index?*
Ans: Since it is a ratio, it has no unit

*(vii) What is total internal reflection?*
Ans: If the angle of incidence in a denser medium is greater
than the critical angle the ray is reflected internally. This is known as total
internal reflection.

*(viii) Aim of refraction through a prism experiment*
(i) To find the angle of a prism (A)

(ii) To study the variation of the angle of deviation (d)
with the angle of incidence (i) and hence to determine the angle of minimum
deviation of the prism by drawing i – d curve.

(iii) To calculate the refractive index of the material of
the prism.

*(ix) Apparatus of refraction through a prism experiment*
Prism, pins, drawing board, drawing paper, etc

*(x) Theory of refraction through a prism experiment*
The refractive index of the material of the prism is given
by, μ
= [sin(A + D)/2]/ sin A/2; where A is the angle and D is the angle of
minimum deviation of the prism.

*(xi) Procedure to find the angle of the prism (A)*
A drawing paper is fixed on a horizontal drawing board. The
outline ABC of the prism is drawn on the paper. The prism is removed. Two
parallel lines are drawn at a convenient distance apart to meet the refracting
faces AB and AC of the prism. Two pins P

_{1}and Q_{1}are fixed on the line drawn to the face AB. The prism is placed back to its position. Two other pins R_{1}and S_{1}are fixed so that they are in line with the reflected images of P_{1}and Q_{1}at the face AB. Similarly two pins P_{2}and Q_{2}are fixed on the other line. The pins R_{2}and S_{2}are fixed so that they are in line with the reflected images of P_{2}and Q_{2}at the face AC. The prism and the pins are removed. The reflected rays R_{1}S_{1}and R_{2}S_{2}are drawn to meet at O. The angle S_{1}O S_{2}is measured. It gives 2A, twice the angle of the prism. From this A, the angle of the prism is found out. The experiment is repeated and the mean angle A of the prism is determined.

*(xii) Procedure to determine the angle of minimum deviation D*
A drawing paper is fixed on a horizontal drawing board. The
prism is placed on it and its outline ABC is traced on the paper. AB and AC are
the refracting faces and BC is the base of the prism. The prism, is removed. At
a convenient point on AB a normal NN' is drawn to AB. A line PQ is drawn making
an angle i, say, 30

^{o}with the normal. Two pins P_{1}and P_{2}are fixed on this line. The prism is placed on the paper at its position. Looking through the face AC two other pins P_{1}’; and P_{2}’ are fixed in line with images of P_{1}and P_{2}. The prism and pins are removed from the paper. A straight line RS is drawn passing, through the positions of the pins P_{1}’ and P_{2}’. Then RS is the emergent ray corresponding to the incident ray PQ. The incident ray PQ and the emergent ray RS are produced to meet at O. The angle of deviation d is measured. The experiment is repeated for angles of incidence 35°, 40°, ... 60° and the corresponding angles of deviation are measured. A graph is drawn with i along the X-axis and d along the Y-axis. The deviation at the lowest point on the graph gives the angle of minimum deviation D.

*(xiii) Procedure to calculate the refractive index of the material of the prism*
The angle A and the angle of minimum deviation D of the
prism are determined as explained above. The refractive index of the material
of the prism is calculated using the relation,

N = [sin(A + D)/2] /sin A/2

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