Chapter 9 · Light - Reflection and Refraction

NCERT Class 10 Science Chapter 9 Exercises

All 17 questions of NCERT Class 10 Science Chapter 9 Exercises (Light - Reflection and Refraction), in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Which one ​of the following materials cannot ​be ​used to ​make ​a lens?

   1. (a)Water
   2. (b)Glass
   3. (c)Plastic
   4. (d)Clay
   Show answer

   (d) Clay

   Step-by-step solution →
2. Q2
   The image formed by a concave mirror is ​observed ​to be virtual, erect and ​larger ​than ​the object. Where should ​be the position of the object?

   1. (a)Between the principal focus and the centre of curvature
   2. (b)At the centre of curvature
   3. (c)Beyond the centre of curvature
   4. (d)Between the pole of the mirror and its principal focus.
   Show answer

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

   Step-by-step solution →
3. Q3
   Where should an object be placed in front of ​a convex ​lens to ​get a ​real image of the ​size ​of the ​object?

   1. (a)At the principal focus of the lens
   2. (b)At twice the focal length
   3. (c)At infinity
   4. (d)Between the optical centre of the lens and its principal focus.
   Show answer

   (b) At twice the focal length

   Step-by-step solution →
4. Q4
   A spherical ​mirror and ​a thin ​spherical lens ​have ​each ​a ​focal length of $-15$ cm. The mirror and the ​lens are likely to be

   1. (a)both concave.
   2. (b)both convex.
   3. (c)the mirror is concave and the lens is convex.
   4. (d)the mirror is convex, but the lens is concave.
   Show answer

   (a) both concave.

   Step-by-step solution →
5. Q5
   No matter how ​far you stand ​from a ​mirror, your image appears ​erect. The ​mirror is ​likely to be

   1. (a)only plane.
   2. (b)only concave.
   3. (c)only convex.
   4. (d)either plane or convex.
   Show answer

   (d) either plane or convex.

   Step-by-step solution →
6. Q6
   Which of the ​following ​lenses would you ​prefer ​to ​use ​while reading small letters ​found ​in a dictionary?

   1. (a)A convex lens of focal length $50$ cm.
   2. (b)A concave lens of focal length $50$ cm.
   3. (c)A convex lens of focal length $5$ cm.
   4. (d)A concave lens of focal length $5$ cm.
   Show answer

   (c) A convex lens of focal length $5$ cm.

   Step-by-step solution →
7. Q7
   We wish to obtain ​an ​erect image ​of an object, using a concave mirror of focal ​length $15$ cm. What should be the range ​of distance of ​the object from the mirror? What is the nature of ​the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation ​in this case.
   Show answer

   The object must be nearer than 15 cm (between pole and focus); the image is virtual, erect and larger than the object.

   The object must be placed at a distance of less than 15 cm from the mirror, that is anywhere between the pole and the principal focus. The image is then virtual and erect, and it is formed behind the mirror. It is larger than the object. For example, with the object at 10 cm the mirror formula gives $v=+30$ cm and $m=-\frac{v}{u}=+3$, so the image stands 30 cm behind the mirror, upright and three times as tall as the object.

   Step-by-step solution →
8. Q8
   Name the type ​of mirror ​used in the following situations.

   1. (a)Headlights of a car.
   2. (b)Side/rear-view ​mirror ​of a vehicle.
   3. (c)​Solar furnace. Support ​your ​answer with reason.
   Show answer

   Car headlights and solar furnaces use concave mirrors; rear-view mirrors use convex mirrors for a wider field of view.

   (a) Concave mirror. The bulb is placed at its principal focus, so the light reflected from the mirror leaves as a parallel beam that travels far down the road without spreading. (b) Convex mirror. It always forms a virtual, erect and diminished image, whatever the distance of the vehicle behind, and it has a much wider field of view than a plane mirror of the same size, so the driver sees more of the traffic behind. (c) Concave mirror. A large concave mirror reflects the parallel rays of sunlight through its focus, concentrating the energy collected over its whole surface onto a small target placed there, which reaches a very high temperature.

   Step-by-step solution →
9. Q9
   One-half ​of a convex lens is ​covered with a black paper. Will this lens ​produce ​a complete image ​of the object? Verify your ​answer experimentally. Explain your ​observations.
   Show answer

   Yes, it still forms a complete image of the same size and position, only fainter, since the remaining rays still reach every image point.

   Yes. The lens still produces a complete image of the object, in the same position and of the same size as before, but the image is fainter. Light from every point of the object falls on the whole face of the lens, and every one of those rays is refracted to the same image point, so covering half the lens removes some of the rays that build each image point but never removes an image point itself. Experimentally, a lighted candle and a screen give a sharp inverted image; covering the upper half of the lens, and then the lower half instead, leaves the same complete image on the screen both times, only dimmer. The brightness falls to about half because only half the area of the lens is now collecting light, while the position and size are fixed by $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ and $m=\frac{v}{u}$, in which the exposed area does not appear.

   Step-by-step solution →
10. Q10
    An object $5$ cm in ​length is held $25$ ​cm ​away from a ​converging lens of focal length $10$ cm. Draw the ray ​diagram and find the ​position, size ​and the ​nature of the image formed.
    Show answer

    $v=+16.7$ cm: the image is real and inverted, 3.3 cm long and 16.7 cm from the lens on the side opposite the object.

    The image is formed $16.7$ cm from the lens, on the side opposite the object. It is $3.3$ cm in size — reduced to two-thirds of the object — and it is real and inverted.

    Step-by-step solution →
11. Q11
    A concave lens of focal length $15$ cm forms an image $10$ ​cm from ​the lens. How far ​is ​the ​object placed from the ​lens? Draw ​the ray diagram.
    Show answer

    The object is 30 cm from the lens on the same side as the image ($u=-30$ cm); the image is virtual, erect and diminished.

    The object is placed $30$ cm from the lens, on the same side as the image (in the sign convention, $u=-30$ cm). The image is virtual, erect and one-third the size of the object.

    Step-by-step solution →
12. Q12
    An object ​is ​placed at a distance of $10$ cm from ​a ​convex ​mirror of focal length $15$ cm. Find the position ​and ​nature of the image.
    Show answer

    $v=+6.0$ cm: the image is 6.0 cm behind the mirror, virtual, erect and diminished ($m=+0.6$).

    The image is formed $6.0$ cm behind the mirror. It is virtual and erect, and diminished to $0.6$ times the size of the object.

    Step-by-step solution →
13. Q13
    The ​magnification ​produced by a ​plane ​mirror is $+1$. ​What does ​this mean?
    Show answer

    The image is the same size as the object, virtual and erect, and as far behind the mirror as the object is in front.

    $m=1$ indicates that the image formed by a plane mirror is exactly the same size as the object; the positive sign indicates that the image is virtual and erect. Since $m=-v/u$, it also follows that $v=-u$, so the image stands as far behind the mirror as the object stands in front of it.

    Step-by-step solution →
14. Q14
    An ​object $5.0$ ​cm in length is placed at ​a distance of $20$ cm ​in front of a convex mirror ​of ​radius of curvature $30$ cm. Find the position of ​the image, its nature and size.
    Show answer

    The image is $8.6\text{cm}$ behind the mirror, virtual and erect, and reduced to $2.2\text{cm}$ ($m=+0.43$).

    The image is formed $8.6$ cm behind the mirror. It is virtual and erect, and its size is $2.2$ cm — reduced, since the magnification is $+0.43$.

    Step-by-step solution →
15. Q15
    An object of ​size $7.0$ cm is placed at $27$ cm in ​front of a ​concave mirror of focal length $18$ cm. At what distance from the ​mirror ​should ​a screen be placed, so ​that a ​sharp focussed image can be obtained? Find the size and the nature of the image.
    Show answer

    The screen is 54 cm from the mirror on the object side; the image is real, inverted, magnified two times, 14 cm tall.

    The screen should be placed $54$ cm from the mirror, on the object side — the same side as the object (in the sign convention, $v=-54$ cm). The image obtained there is $14$ cm in size — magnified two times — and it is real and inverted.

    Step-by-step solution →
16. Q16
    Find ​the focal ​length of a lens of ​power $-2.0$ D. ​What ​type of ​lens is ​this?
    Show answer

    $f=-0.50$ m (that is $-50$ cm); the negative sign shows a concave, diverging lens.

    The focal length is $-0.50$ m, that is $-50$ cm. The negative sign shows that this is a concave lens — a diverging lens.

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17. Q17
    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?
    Show answer

    $f=+0.67$ m (about 67 cm); the lens is convex, so it is converging.

    The focal length is $+0.67$ m, that is about $67$ cm. The positive sign shows that the prescribed lens is a convex lens, so it is a converging lens.

    Step-by-step solution →