Refraction through a Prism Viva Questions
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 P1 and Q1 are
fixed on the line drawn to the face AB. The prism is placed back to its
position. Two other pins R1 and S1 are fixed so that they
are in line with the reflected images of P1 and Q1 at the
face AB. Similarly two pins P2 and Q2 are fixed on the
other line. The pins R2 and S2 are fixed so that they are
in line with the reflected images of P2 and Q2 at the
face AC. The prism and the pins are removed. The reflected rays R1 S1
and R2 S2 are drawn to meet at O. The angle S1
O S2 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, 30o with the normal. Two pins P1 and P2
are fixed on this line. The prism is placed on the paper at its position. Looking
through the face AC two other pins P1’; and P2’ are fixed
in line with images of P1 and P2. The prism and pins are
removed from the paper. A straight line RS is drawn passing, through the
positions of the pins P1’ and P2’. 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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