## 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

## Concave Lens Viva Questions

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 I1 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 I2 of the screen is noted. With respect to the concave lens if an object is placed at I2, a virtual image will be obtained at I1. Hence LI2 can be taken as u and LI1 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

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 cross-wire 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

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/r1 – 1/r2) , where r1 and r2 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 /v-graph?

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 m1 and m2 are the magnifications in the two postions what is the relation between these?

Ans: m1 x m2 = 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 = (D2 – d2)/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. u-v 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. u-v 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. 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 L1 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 L2 of the lens is noted. The distance between the positions L1 and L2 is measured as d. The focal length of the lens is calculated from the equation,
f = (D2 — d2)/4D.
The experiment is repeated for different values of D and the average value of f is calculated.

## Concave Mirror Viva Questions

Concave Mirror Experiment Viva Questions
(i) Distinguish between real and virtual images

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

The u and v are the object distance and the image distance from the mirror,
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. u-v 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. u-v 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 X-axis and 1/v along the Y-axis taking 0-0 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.