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Class Ten Science Science NCERT Solutions Chap 10

CHAPTER 10: LIGHT: REFLECTION AND REFRACTION

Q.26. 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.

Ans. Focal length of convex mirror, f = +15 cm

Object distance, u = – 10 cm

According to the mirror formula,

The positive value of vindicates that the image is formed behind the mirror.

Magnification,

Q.27. The magnification produced by a plane mirror is +1. What does this mean?

Ans. The positive sign means an 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.

Q.28. An object 5 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position, nature and size of the image.

Ans. Object distance, u = – 20 cm

Object height, h = 5 cm

Radius of curvature, R = 30 cm

Radius of curvature = 2 × Focal length

R = 2f

f = 15 cm

According to the mirror formula,

v = 8.57 cm

The positive value of ‘v’ indicates that the image is formed behind the mirror.

Magnification,

The positive value magnification indicates that the image formed is virtual.

Magnification,

h′ = m × h = 0.425 × 5 = 2.14 cm

The positive value of image height indicates that the image formed is erect.

Therefore, the image formed is virtual, erect, and smaller in size.

Q.29. 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 sharply focused image can be obtained? Find the size and nature of the image.

Ans. Object distance, u = – 27 cm

Object height, h = 7 cm

Focal length, f = – 18 cm

According to the mirror formula,

v = – 54 cm

The screen should be placed at a distance of 54 cm in front of the given mirror.

Magnification,

The negative value of magnification indicates that the image formed is real.

Magnification,

The negative value of image height indicates that the image formed is inverted.

Q.30. 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 = – 2 D.

f = – 1/2 = – 0.5 m.

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

Q.31. 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.

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