A three times magnified virtual image of an object kept at a distance

1.	A three times magnified virtual image of an object kept at a distance of 24 cm from a convex lens is formed focal length of the lens is A) 72 B) 24C) 36 D) -12
Answer» The correct option is D) -12. Here Magnification "M" = 3 u= -8 cm as u (object distance) is always negative. M = -v/u 3 = -v/-8 3 = v/8 v (position of image)= 24 cm The image is formed in front of the mirror. By using lens mirror formula, focal length can be calculated. For focal length: 1/f = 1/u +1/v 1/f = 1/-8 + 1/24 1/f = 1/24 - 1/8 1/f =1/24 - 3/24 1/f = -2/24 = -1/12 f= -12 cm Hence the focal length of the lens is - 12 cm.

A three times magnified virtual image of an object kept at a distance of 24 cm from a convex lens is formed focal length of the lens is A) 72 B) 24C) 36 D) -12

Answer»

The correct option is D) -12.

Here Magnification "M" = 3

u= -8 cm as u (object distance) is always negative.

M = -v/u

3 = -v/-8

3 = v/8

v (position of image)= 24 cm

The image is formed in front of the mirror.

By using lens mirror formula, focal length can be calculated.

For focal length:

1/f = 1/u +1/v

1/f = 1/-8 + 1/24

1/f = 1/24 - 1/8

1/f =1/24 - 3/24

1/f = -2/24 = -1/12

f= -12 cm

Hence the focal length of the lens is - 12 cm.