Chapter 9

NCERT
Class 10
Science
Solutions
15. An object of size 7.0 cm is placed at a distance of 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 clear focused image of the object is formed?

Question:

An object of size 7.0 cm is placed at a distance of 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 clear focused image of the object is formed? Find the size and nature of an object.

Answer:

Height of an object h1 = 5 cm

Object distance u = – 27 cm

Focal length = – 18 cm

Height of an image h1 = ?

Image distance u = ?

From the formula :

⇒ 1/u + 1/v = 1/f

⇒ 1/(-27) + 1/v = 1/(-18)

⇒ 1/v = 1/18 + 1/27

⇒ - (3+2)/ 54

⇒ -1/54

⇒ v = –54 cm

Therefore, screen should be placed at a distance of 54 cm from the screen.

m = h2/h1 = - v/u

⇒ h2/7 = -54/-27

⇒ h2/7 = 2

⇒ h2 = 14 cm

Thus, the height of the image formed is 14 cm and is real and inverted in nature.

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