NCERT Solutions Exercise
Q1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay
Ans: (d) Clay
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?
(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: (a) Between the principal focus and the centre of curvature
Q3. 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
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
(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.
Q5. 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.
Q6. 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.
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.
Answer: In a concave mirror, a virtual and erect image is formed only when the object is between the principal focus and the pole. Since the focal length of a concave mirror is 15 cm, that is, the distance between the pole and the focus is 15 cm. Therefore the object should be placed between 0cm to 15cm in front of the mirror, only then a erect image is formed.
Nature of Image : Virtual and Upright.
Image size: larger than the object.
Q8. 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.
Ans:
(a) The head-light of a car is made of a concave mirror, because if the bulb is placed at the principal focus of the mirror, it reflects from the mirror and forms a parallel beam.
(b) A convex mirror is used for the side/rear-view mirror of a vehicle because it always produces a small but erect image. Since the convex mirror is curved outwards, its field of vision is greatly increased, so that the driver can see a large part of the rear of the vehicle.
(c) Sunlight has to be focused in a solar furnace, for which a concave mirror is suitable. This mirror passes light rays coming through infinity parallel to the principal axis through the focus, due to which the temperature around the focus increases from 1800C to 2000C.
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.
Answer: Yes, even if half of a convex lens is covered with black paper, this lens forms a complete and clear image of a given object.
Testing - Now keep a convex lens covered in half with a black paper with the help of a stand and keep a burning candle on one side of the lens and a white curtain on the other side. In observation we find that the full image of the candle is made on the screen which is real and inverted.
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.
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.
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.
Q13. The magnification produced by a plane mirror is +1. What does this mean?
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.
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.
Q16. Find the focal length of a lens of power – 2.0 D. What type of lens is this?
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?
