NCERT Solutions for Class 10th Science

Chapter 10 – LIGHT – REFLECTION AND REFRACTION

(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)

 

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

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

Ans     We all know that Radius of Curvature(R) = 2 x focal length(f)

            Here R = 20 cm, So f = 20/2 = 10 cm.

            Therefore, focal length of spherical mirror is 10 cm.

Q.3      Name a mirror that can give an erect and enlarged image of an object.

Q.4      Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Ans     Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they                   have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view                 much larger area than would be possible with a plane mirror.

Q.5      Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Ans     We all know that Radius of Curvature(R) = 2 x focal length(f)

            Now R = 32 cm, So f = 32/2 = 16 cm.

            Therefore, focal length of convex mirror is 16 cm.

Q.6      A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in                  front of it. Where is the image located?

Q.7      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?

Q.8      Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The                    speed of light in vacuum is 3 × 108 m/s.

Q.9      Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest               optical density.

Q.10      You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the                    information given in Table 10.3.

Ans      Refractive index is inversely proportional to speed of light i.e. Refractive index α (1/speed of light). Hence                the light will travel fastest in water as it has the lowest refractive index (1.33) among the three.

Q.11      The refractive index of diamond is 2.42. What is the meaning of this statement?

Q.12      Define 1 dioptre of power of a lens.

Q.13      A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the                    needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the                      power of the lens.

Q.14      Find the power of a concave lens of focal length 2 m.

 

EXERCISES

 

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

           (a) Water     (b) Glass     (c) Plastic     (d) Clay

Ans     (d) Clay

            Because clay does not allow light to pass through it.

Q.2      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?

           (a) Between the principal focus and the 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.

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

Q.3      Where should an object be placed in front of a convex lens to get a real image of the size of the object?

           (a) At the principal focus of the lens

           (b) At twice the focal length

           (c) At infinity

           (d) Between the optical centre of the lens and its principal focus.

Ans    (b) At twice the focal length

Q.4      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

          (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.

Ans    (a) both concave.

Q.5      No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be

           (a) plane.

           (b) concave.

           (c) convex.

           (d) either plane or convex.

Ans    (d) either plane or convex.

Q.6      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.

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

Q.7      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.

Q.8      Name the type of mirror used in the following situations.

           (a) Headlights of a car.

           (b) Side/rear-view mirror of a vehicle.

           (c) Solar furnace.

           Support your answer with reason.

Q.9      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.

Q.10     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.

Q.11      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.

Q.12      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.

Q.13      The magnification produced by a plane mirror is +1. What does this mean?

Q.14      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.

Q.15      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.

Ans      Height of object, h = 7 cm

             Object distance, u = - 27 cm

             Focal length, f = - 18 cm

             From mirror formula, we have

             1/f = 1/v + 1/u 

             1/-18 = 1/v + 1/-27 

              » v = - 54 cm

              Now magnification, m = h’/h = -v/u = - (- 54 / -27)  = - 2

              » h’ = - 2 X 7  = - 14 cm (-ve sign indicates that the image is inverted)

              Hence, size of image – 14 cm

              Nature of image – Enlarged, real and inverted.           

Q.16      Find the focal length of a lens of power – 2.0 D. What type of lens is this?

Ans       Power, P = - 2.0 D

              Hence focal length, f(metres) = 1/P = 1/-2 = - 0.5 m = - 50 cm

              Since the focal length is negative, it is a concave lens.

Q.17      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, P = + 1.5 D

              Hence focal length, f(metres) = 1/P = 1/1.5 = 0.67 m = 67 cm

              Since the focal length is positive, it is a convex lens i.e. converging lens.