SOLUTION OF EXERCISE

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 lens is formed from transparent objects like water, glass, plastic. But not clay because it is an opaque medium)

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. ( because at this position, the image formed in a concave mirror is virtual, erect and enlarged and the position of the image is behind the mirror.

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

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: both concave (because both for concave mirror and for convex lens the focal length is taken as –ve)

5. 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.

5. 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.

Ans: (d) either plane or convex. (Because for these two mirrors we get only erect images)

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 ( because convex lens gives an enlarged image of any object, and also as the power of a lens is inversely proportional to the focal length hence the lens having lesser focal length has more power then a lens having greater focal length)

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.

Ans: We know that,

• In a concave mirror, erect image is formed, when the object is between the focus and the pole of the mirror.
• And also we measure all the distance from pole of spherical mirror. Hence the pole is taken as origin.
• Hence the range of the position of the object is taken 0-15cm
• The image is larger than the object.
• Nature of the image is virtual and erect.

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

• Concave mirrors are used in the headlights of car
• Because concave mirror is used to obtain powerful parallel beams, which helps the car driver to see the roads in-front of his car.

b) side/rear view mirrors a vehicle:

• Convex mirror are used as side/rear view mirror in vehicle.
• Because convex mirror always forms diminished image of objects irrespective of the position of the object.
• Hence it covers a large area, than the others. Hence it is used as rear view mirror.

ANS: d) clay (because lens is formed from transparent objects like water, glass, plastic. But not clay because it is an opaque medium)

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. ( because at this position, the image formed in a concave mirror is virtual, erect and enlarged and the position of the image is behind the mirror.

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

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: both concave (because both for concave mirror and for convex lens the focal length is taken as –ve)

5. 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.

5. 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.

Ans: (d) either plane or convex. (Because for these two mirrors we get only erect images)

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 ( because convex lens gives an enlarged image of any object, and also as the power of a lens is inversely proportional to the focal length hence the lens having lesser focal length has more power then a lens having greater focal length)

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.

Ans: We know that,

• In a concave mirror, erect image is formed, when the object is between the focus and the pole of the mirror.
• And also we measure all the distance from pole of spherical mirror. Hence the pole is taken as origin.
• Hence the range of the position of the object is taken 0-15cm
• The image is larger than the object.
• Nature of the image is virtual and erect.

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

• Concave mirrors are used in the headlights of car
• Because concave mirror is used to obtain powerful parallel beams, which helps the car driver to see the roads in-front of his car.

c) Solar furnace:

• Concave mirror is used in solar furnace.
• Because it concentrates the parallel rays those come from the sun. Hence generates more heat.
• 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.

Ans:

• Every part of the lens casts image. Hence, if one half of a convex lens is covered with black paper, the full image of the object is created.
• But the image of the object is not so clearly visible. Because in the picture the dotted light rays are not refracted.
• Every part of the lens casts image. Hence, if one half of a convex lens is covered with black paper, the full image of the object is created.
• But the image of the object is not so clearly visible. Because in the picture the dotted light rays are not refracted.