Refraction through a Prism Viva Questions
February 23, 2020

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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 Xaxis and d along the Yaxis. 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
Concave Lens Viva Questions
February 20, 2020

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Concave Lens Viva Questions with Answers
(i) Why is a concave lens known as a diverging lens?
Ans: When a parallel beam of light is incident on a concave
lens the beam emerges as a divergent beam. Hence the lens is called a diverging
lens.
(ii) What is the nature of the image formed when an
object is placed in front of a concave lens?
Ans: The images, virtual, erect diminished and is formed
within the focus of the lens
(iii) In the experiment to find the focal length of a
concave lens by combination method the focal length of the convex lens must be
smaller than that of the concave lens. Why?
Ans: So that the combination behaves as a convex lens and
forms real images.
(iv) A convex lens of focal length +10 cm and a
concave lens of focal length 10 cm are brought in contact. What is the focal
length of the combinations?
Ans: Infinity
v) How will you distinguish between a convex lens and
a concave lens?
Ans: The lens is placed close to a printed matter. If the
image is magnified the lens is convex. If the image of the print is diminished
the lens is concave.
vi) Can we form magnified image with a concave lens.
Ans: No. The concave lens can form only diminished images.
vii) Aim of the Concave Lens Experiment
Ans: To find the focal length of the given concave lens
using an auxiliary convex lens (i) in contact and (ii) out of contact.
viii) Apparatus of the Concave Lens Experiment
Illuminated wire gauze, concave lens, short focused convex
lens, screen, lens holder, etc.
ix) Theory of the Concave Lens Experiment
Ans:
(1) Combination method (Lenses in contact):
If F is the
focal length of the combination of a convex lens of focal length f' and concave
lens, of local length f , then,
1/F = 1/f’ + 1/f
Therefore, f = Ff’/(f’  F)
(2) Auxiliary convex lens method (Lenses out of contact)
If u is the object
distance and v is the virtual image distance from the concave lens.
f = uv/(v  u)
x) Procedure of the Concave Lens Experiment
Ans:
(1). Lenses in contact (combination method)
The focal length (f’) of the convex lens is found out by
displacement method or u  v, method as described in previous blog post. The
convex lens is then kept in contact with the concave lens and focal length (F)
of the combination is found out by u – v method. If f is the focal length of
the concave lens, then,
1/F = 1/f’ + 1/f
Therefore, f = Ff’/(f’  F)
(2). Lenses out of contact (Auxiliary convex lens method)
The convex lens is placed in front of the illuminated wire
gauze and the screen is adjusted on the other side of the lens so that a clear
image of the wire gauze is obtained on the screen. The position I_{1}
of the screen is noted. The concave lens L is then interposed between the
convex lens and the screen. The image becomes blurred. The screen is then moved
away from the lens until a clear image is obtained on it. The position I_{2}
of the screen is noted. With respect to the concave lens if an object is placed
at I_{2}, a virtual image will be obtained at I_{1}. Hence LI_{2}
can be taken as u and LI_{1} as v. Then,
1/f = 1/u – 1/v
Therefore, f = uv/ (v  u)
The experiment is repeated for different positions of the
lens and the mean value of f is found out.
Convex Mirror Viva Questions
February 17, 2020

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Convex Mirror Experiment Viva Questions with Answers
(i) Define principal focus of a convex mirror.
Ans: Paraxial rays parallel to the principal axis after
reflection at the convex mirror appear to diverge from a point. This point is
the principal focus.
(ii) What is radius of curvature of a convex mirror?
Ans: It is the radius of the sphere of which the mirror
forms a part.
(iii) What is the relation between radius of curvature and
focal length?
Ans: r = 2f
(iv) An object is placed in front of a convex mirror. What
is the nature of the image formed?
Ans: The image is virtual, erect diminished and formed with
in the focus.
(v) What is the use of convex mirror?
Ans: It is used as rear view mirror for motor car. It offers
wide field of view and the image is erect.
(vi) What is the radius of curvature of a plane mirror?
Ans: Infinity
(vii) Aim of Convex Mirror Experiment
To find the focal length of a convex mirror using a convex
lens
(viii) Apparatus of Convex Mirror Experiment
Illuminated object (light box with a cross wire or wire
gauze), convex mirror, convex lens, stands for mounting the mirror and lens,
screen etc.
(ix) Theory of Convex Mirror Experiment
If r is the radius of curvature of the convex mirror, its
focal length, f = r/2
(x) Procedure of Convex Mirror Experiment
The convex lens is placed in front of the illuminated
object, the light box with a cross wire. A screen is adjusted until a clear
image is formed on it. The convex mirror is now placed between the lens and the
screen with its reflecting surface towards the lens. The mirror is adjusted in
position until a clear image of the crosswire is obtained on the surface of
the light box side by side with the cross wire. The distance between the
position of the convex mirror and the screen is measured. This gives the radius
of curvature r of the mirror. The experiment is repeated with the illuminated
objects at different distances from the lens and the average value of r is
found out. Hence, f = r/2 is calculated.
Convex Lens Viva Questions with Answers
February 15, 2020

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Convex Lens Viva Questions with Answers
(i) What is the minimum distance between the object
and its image for a convex lens?
Ans: 46
(ii) Define focal length of a convex lens
Ans: It is the distance between optic centre and principal
focus
(iii) What is lens makers formula?
Ans: 1/f = (n — 1)(1/r_{1} – 1/r_{2}) , where
r_{1} and r_{2} are the radii of curvature of the two faces.
(iv) What is meant by power of a lens?
Ans: It is the reciprocal of the focal length expressed in
metres, p = 1/f
(v) What is the unit of power?
Ans: Dioptre
(vi) Which type of lens is used in microscopes?
Ans: Convex lenses
(vii) What is the shape of 1/u — 1 /vgraph?
Ans: The graph is straight line with equal intercepts
(viii) Are the positions of the object and image interchangable
Ans: Yes, for real images only
(ix) In the displacement method we get magnified and
diminished images for two postions of the lens between the object and screen.
If m_{1} and m_{2} are the magnifications in the two postions
what is the relation between these?
Ans: m_{1} x m_{2} = 1
(x) Can a convex lens form virtual image?
Ans: Yes, when the object is between F and C
(xi) What is the property of the optic centre?
Ans: A ray passing through the optic centre passes without
deviation
(xii) Aim of Convex Lens Experiment
Ans: To find the focal length of the given convex lens
(xiii) Apparatus of Convex Lens Experiment
Ans: Convex lens, screen, illuminated wire gauze, etc.
(xiv) Theory of Convex Lens Experiment
Ans: If u ad v are the object distance and the image
distance from the convex lens, its focal length f = uv/(u + v). If D is the
distance between the object and the screen, and d is the distance between the
conjugate positions of the lens,
f = (D^{2} – d^{2})/4D
(xv) Procedure of Convex Lens Experiment
Ans: The focal length of the convex lens can be found out by
different methods.
1. Distant object method
The convex lens is
faced to a distant scenery and the position of the screen is adjusted so that a
well defined image of the scenery is formed on the screen. The distance between
the lens and the image on the screen gives the focal length of the lens. The
experiment is repeated and the mean focal length is found out.
2. uv method
The convex lens is placed in front of the illuminated wire
gauze. A screen is adjusted on the other side of the lens so that a clear image
of the object is formed on it. The distances of the object and the image from
the lens are measured as u and v. The focal length is then calculated using the
formula,
f = uv/(u + v)
The experiment is repeated for different values of u.
Readings are taken for both magnified and diminished images.
3. uv graph
A graph is drawn with u along the Xaxis and v along the
Yaxis taking a common origin and same scale for both axes. A bisector to the
angle XOY is drawn which meets the graph at P. Then OA = OB =2f. Hence the
focal length can be found out.
4. 1/u  1/v graph
A graph is drawn with 1/u along the X axis and 1/v along the
Y axis taking zero as origin and same scale for both axes. The graph is a
straight line intercepting the axes at A and B. Then OA = OB = 1/f; from which
f can be calculated.
5. Displacement method
The screen is placed at a distance more than 4f from the
object, the illuminated wire gauze. The distance between the object and the
screen is measured as D. The convex lens is placed nearer to the object in
between the object and the screen. Its position is adjusted to get well defined
enlarged image of the object on the screen. The position L_{1} of the
lens is noted. The lens is moved towards the screen till a well defined
diminished image of the object is obtained on the screen. The position L_{2}
of the lens is noted. The distance between the positions L_{1} and L_{2}
is measured as d. The focal length of the lens is calculated from the equation,
f = (D^{2} — d^{2})/4D.
The experiment is repeated for different values of D and the
average value of f is calculated.
Concave Mirror Viva Questions
February 06, 2020

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Concave Mirror Experiment Viva Questions
The u and v are the object distance and the image distance
from the mirror,
Ans: Rays proceeding from an object after
reflection or refraction converge to a point. The image formed is real. If the
rays appear to diverge from a point, the image is virtual. Real images can he
caught on a screen. Virtual images cannot be obtained on a screen
(ii) Define principal focus of a concave mirror
Ans: Paraxial rays parallel to the principal axis after
refraction converge to a point. That point is the principal focus.
(iii) What is the position of the object for which we
get enlarged virtual image?
Ans: Object must be between focus and pole of the mirror
(iv) At what distance from a concave mirror should an
object be placed so that its image is formed at the same place?
Ans: At the centre of curvature of the concave mirror
(v) What is the shape of u.v. graph?
Ans: It is a part of a hyperbola
(vi) Define centre of curvature of a concave mirror
Ans: It is the centre of the sphere of which the mirror
forms a part.
(vii) The aperture of the mirror is small. Why?
Ans: To reduce spherical aberration
(viii) If the radius of curvature of a concave mirror
is 20 cm. What is its focal length?
Ans: Focal length = 10 cm
(ix) What type of mirror do we use to avoid spherical
aberration?
Ans: Paraboloidal mirror
(x) For real images formed by a concave mirror, are
the positions of the object and image interchangeable
Ans: Yes
(xi) Aim of Concave mirror Experiment
Ans: To find the focal length of the given concave mirror.
(xii) Apparatus of Concave mirror Experiment
Ans: Concave mirror, screen, illuminated wire gauze, etc.
(xiii) Theory of Concave mirror Experiment
f = uv/(u + v)
(xiv) Procedure of Concave mirror Experiment
The focal length of the concave mirror can be found out by
different methods.
1. Distant object method
The concave mirror is faced to a distant scenery. The
position of the screen is adjusted so that a clear image of the scenery is
obtained on it. The distance between the mirror and the screen is the focal
length. The experiment is repeated three times and the mean value is found out.
2. Normal incidence method
A piece of wire gauze fitted in a hole on the side of a
wooden box and illuminated by an electric lamp serves as the object. The
concave mirror is placed in front of the object and its position is adjusted so
that a clear image of the wire gauze is formed by the side of the object. The
distance between the object and the mirror is the radius of curvature r of the
mirror. Half the radius of curvature is calculated as focal length. The
experiment is repeated and the mean value of the focal length f of the mirror
is found out.
3. uv method
The illuminated wire gauze, the object, is placed in front
of the concave mirror at a distance more than the focal length (f) of the
mirror. A screen is placed in front of the mirror and its position is so adjusted
that a clear image of the wire gauze is formed on it. The distances of the
object and image to the mirror are measured as u and v. Then the focal length
of the mirror can be found out using the formula, f = uv/(u + v). The
experiment is repeated for different distances. Readings are taken for both
magnified and diminished images.
4. uv graph
A graph is drawn with u along the X axis and v along the Y
axis taking a common origin and same scale for both axes. A bisector to the
angle XOY is drawn which meets the graph at P. The coordinates of P is the
radius of curvature. Thus OA = OB = 2f . From this the focal length f is found
out.
5. 1/u  1/v graph
A graph is drawn with 1/u along the Xaxis and 1/v along the
Yaxis taking 00 as origin and same scale for both axes. The graph is a
straight line intercepting the axes at A and B. Then OA = OB = 1/f. Hence f
can be calculated.
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