# MP Board Class 10th Science imp questions Chapter 10 Light Reflection and Refraction

CHAPTER   10

Light-Reflection and Refraction

Objective Questions

Multiple Choice Questions

1. To determine the approximate value of the focal length of a given concave mirror, you focus the image of a distant object formed by the mirror on a screen. The image obtained on the screen, as compared to the object is always

(a) Laterally inverted and diminished

(b) Inverted and diminished

(c) Erect and diminished

(d) Erect and highly diminished

1. A student obtains a blurred image of a distant object on a screen using a convex lens. To obtain a distinct image on the screen, he should move the lens

(a) away from the screen

(b) towards the screen

(c) to a position very far away from the screen

(d) either towards or away from the screen depending upon the position of the

object.

1. Which of the following materials cannot be used to make a lens?

(a) Water

(d) Clay.

(c) Plastic

(b) Glass

1. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be-

(a) only plane

(b) only concave

(c) only convex

(d) either plane or convex.

1. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?

(a) A convex lens of focal length 50 cm

(b) A concave lens of focal length 50 cm

(c) A convex lens of focal length 5 cm

(d) A concave lens of focal length 5 cm.

1. A spherical mirror and a thin spherical lens each have a focal length of – 15 cm. The mirror and the lens are likely to be :

(a) both concave

(b) both convex

(c) the mirror is concave and the lens is convex

(d) the mirror is convex, but the lens is concave.

1. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. What should be the position of the object?

(a) Between the principal focus and centre of curvature.

(b) At the centre of curvature.

(c) Beyond the centre of curvature.

(d) Between the pole of the mirror and its principal focus.

1. S.I. unit of power of lens is

(a) meter

(b) centimeter

(c) dioptre

(d) kilometer.

Ans. 1. (b), 2. (b), 3. (d), 4. (d), 5. (c). 6. (a), 7. (d), 8. (c).

Fill in the Blanks

1. The geometric centre of the mirror is called

…………..

1. Convex and concave mirrors are collectively known as …………….

1. Virtual image cannot be received on a…………..

1. The SI unit of the power of a lens is……………

1. The image from a convex mirror will always be ……………

1. The radius of curvature of a spherical mirror is………… of its focal length. objects are capable of reflecting light to our eyes.

8…………… objects generate their own light. 9. The focal length of a spherical mirror is….. of its radius of curvature.

Ans. 1. Pole. 2. Spherical mirror, 3. Screen, 4. dioptre, 5. Virtual, 6. twice, 7. Illuminated, 8. Luminous, 9. half.

True/False

1. The speed of light is higher in a rarer medium than a denser medium.

1. The speed of light in vacuum is 3 x 10 ms.

1. The SI unit of power is Dioptre.

1. The power of a concave lens is negative while that of a convex lens is positive.

1. The central point of a lens is its aperture.

Ans. 1. True, 2. False, 3. True, 4. True, 5. False.

Match the Columns

Column“A^                           Column B

1. Convex mirrors (a) Rear view mirrors in

vehicles

1. Object at Infinity. (b) Erect, diminished

image

1. In spherical mirrors. (c) Image formed at

focus

1. Concave mirrors (d) R=2 prime

1. Convex mirrors. (e) Used in torches and

Ans. L -> (b) , 2, (c),3, (d). 4. (c),5. (a)

1. How many images do we see between two plane mirrors ?

Ans. Infinite.

1. Name a mirror that can give an erect and enlarged image of an

Ans. Concave mirror. object?

1. If the magnification m < 0 then the image formed is erect or inverted?

Ans. Inverted.

1. What type of mirrors are used in saloons ? Ans. Plane mirrors.

1. What type of images are formed by a concave mirror ?

Ans. Both virtual and real.

1. When a ray of light passes through the optical centre (O) of a convex or concave lens, how will it emerge ?

Ans. The emergent ray will have no deviation.

1. If the lens is used to form an image of the sun on a screen, how far fromthe lens should the screen be located?

Ans. At its focal length.

1. What is the angle of reflection if a ray falls normally on a plane mirror?

Ans. Zero.

1. A mirror has a focal length – 15 cm, what type of mirror is it?

Ans. Concave mirror, because focal length is negative.

1. Which is optically denser of the two media m_{1} and m_{2} having n = 1 * 71 and 1:36 respectively?

Ans. m_{1} as it has a higher value of refractive index. 11. Write mirror formula.

Ans. 1/f = 1/u + 1/v

1. 1. Define the principal focus of a concave mirror.

Ans. Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principle focus of the concave mirror.

1. 2. The radius of a curvature of a spherical mirror is 20 cm. What is its focal length ?

1. 3. Define 1 dioptre of power of a lens.

Ans. The SI unit of power of lens is dioptre which is denoted by the letter D. 1 dioptre is defined as the power of a lens of focal length 1 metre.

1. 4. Find the power of a concave lens of focal length 2 m.

Ans. Focal length of concave lens, f= 2 m

Power of lens    P=1/f =1/-2

= 0·5 D

1. 5. Find the focal length of a lens of power-2-0 D. What type of lens is this?

Ans. Power of lens,P = 1/f

P=-2D

f-1/2 -0-5 m

A concave lens has a negative focal length. Hence, it is a concave lens.

1. 6. The magnification produced by a plane mirror is +1. What does this mean?

Ans. The positive sign means image formed by a plane mirror is virtual and erect. Since the magnification is 1, it means that the size of the image is equal to the size of the object.

1. 7. A doctor has prescribed a corrective lens of power + 1-5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

Ans. Power of lens,     P = 1/f

P = 1-5 D

f=1/1-5= 10/15 = 0-66 m

A convex lens has a positive focal length. Hence, it is a convex lens or a converging lens.

1. 8. Why do we prefer a convex mirror as a rear view mirror in vehicles ?

Ans. We prefer a convex mirror as a rear view mirror in vehicles because it gives a wider field of view, which allows the driver to see most of the traffic behind him. Convex mirrors always form a virtual, erect and diminished image of the objects placed in front of them.

1. 9. Define the centre of curvature of the spherical mirror.

Ans. Centre of curvature of the spherical mirror : “The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is known as the centre of curvature of the spherical mirror.”

1. 10. Define the radius of curvature of a spherical mirror.

Ans. Radius of curvature of a spherical mirror-“The radius of a spherical shell whose part is the spherical mirror is called the radius of curvature of the spherical mirror.” It is denoted by ‘R’.

1. 11. Define the focal length of a spherical mirror.

Ans. The Focal length of a spherical mirror-The distance between the focus  and pole of a spherical mirror is called focal length of the spherical mirror.” It is denoted by f

1. 12. Define the magnification.

Ans. The Magnification-“Magnification by any optic device is the relative extent which decides that how many times the image formed by the device is magnified than the object. In othe words, “the ratio between the height of the image and the height of the object is called magnification.”

1. 13. Define the power of lens.

Ans. Power of lens-The degree of conversion or diversion of light rays by a lens is defined as power of the lens. Numerically, the power of a lens is equal to the inverse of its focal length expressed in metre” i.e..

1. 1. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

Ans. The ray of light bends towards the normal. When a ray of light enters from an optically rarer medium (having low refractive index) to an optically denser medium (having high refractive, index), its speed slows down and it bends towards the normal Since water is optically denser than air, ray of light entering from air into water will bend towards the normal.

1. 2. Light enters from air to glass having refractive indes 1-50. What is the speed of light in the glass ? The speed of light in vacuum is 3× 10 8m / s

Ans.

1. 3. Name the type of mirror used in the following situations:

(b) side/rear view mirror of a vehicle

(c) solar furnace

Ans. (a) Concave mirror is used in the headlights of a car. This is because concave mirrors can produce powerful parallel beam of b_{2}*br when the light source is placed at their principal focus.

(b) Convex mirror is used as side/rear view mirror of a vehicle because of its

largest field of view, See Ans. to Very Short Answer Type Question No. 8.

(c) Concave mirror is used in a solar furnace because it concentrates the parallel rays of sun at principal focus.

1. 4. The absolute refractive indices of glass and water are 4/3 and 3/2 respectively. If the speed of light in glass is 2 x 10 m/s, calculate the speed of light in (i) vacuum, (ii) water.

Ans.

1. 5. It is desired to obtain an erect image of an object, using concave mirror of focal length of 12 cm.

(1) What should be the range of distance of an object placed in front of the mirror ?

(ii) Will the image be smaller or larger than the object ? Draw a ray diagram to show the formation of image in this case.

(iii) Where will the image of this object be, if it is placed 24 cm in front of the mirror. Draw ray diagram for this situation also to justify your answer.

Show the positions of pole, principal focus and the centre of curvature in the above ray diagrams.

Ans. (1) To obtain an erect image, the object should be placed between the pole and the focus. Here, the focal length of the mirror is 12 cm. Hence, the object should be placed at a distance less than 12 cm.

(ii) The image will be larger than the object (enlarged).

(iii) Since f= 12 cm ⇒ centre of curvature=2f=24 cm. For an object placed at a distance of 24 cm, i.e. at the centre of curvature of a concave mirror, the image formed will be real, inverted and of the same size as that of the object.

1. 6. An object of height 2-5 cm is placed at a distance of 15 cm from the optical centre ‘O’ of a convex lens of focal length 10 cm. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ^ * 0^ * principal focus F and height of the image on the diagram.

Ans. Ray diagram :

1. 7. An object is placed at a distance of 30 cm from a concave lens of focal length 15 cm. List four characteristics (nature, position etc.) of the image formed by the lens.

Ans.

The negative sign of the image distance shows that the image is formed on the left side of the concave mirror. Thus, the image formed by a mirror is virtual, erect and on the same side as the object.

1. 8. A 4 cm tall object is placed on the principal axis of a convex lens. The distance of the object from the optical centre of the lens is 12 cm and its sharp image is formed at a distance of 24 cm from it on a screen on the other side of the lens. If the object is now moved a little away from the lens, in which way (towards the lens or away from the lens) will we have to move the screen to get a sharp image of the object on it again ? How will the magnification of the image be affected?

Ans.

:. The focal length of the lens is 8 cm. Now if the object is moved away from the lens, the screen has to be moved towards the lens. This is because when we move the object away from the lens, the object distance is increased. Hence, by the lens formula, the image distance decreases.

Long Answer Type / Analytical Questions

1. 1. (a) What is meant by power of a lens ? Define its S.I. unit.

(b) You have two lenses A and B of focal lengths +10 cm and -10 cm respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens ? Draw a ray diagram to justify your answer.

Ans.