Question

u ppear when seen from abovey Refractive index ol glass is 15
The image of an object formed by a lensis of magnification -1. If the distance between the one
and its image is 60cm, what is the focal length of the lens? If the object is moved 20cm towards
The lens, where would the image be formed? State reason and also draw a ray diagram in suops
of your answer​

Answer 1

Correct Question

The image of an object formed by a lens is of magnification -1. If the distance between the lens and its image is 60 cm, what is the focal length of the lens? If the object is moved 20 cm towards the lens, where would the image be formed?

Solution

→ m = v/u

• m = - 1 (Here "-" sign shows that image is real which can be only in Convex Lens)
• v = 60 cm (Since image in Convex lens is in positive side)
• u = unknown

→ - 1 = 60/u

→ u = - 60

Hence object is formed at 60 cm from lens (at Center of Curvature) at negative side.

Mirror Formula

→ 1/f = 1/v - 1/u

→ 1/f = 1/(60) - 1/(- 60)

→ 1/f = 1/60 + 1/60

→ f = 30 cm

______________________________

→ u' = 60 - 20 = 40

→ 1/(30) = 1/v - 1/(- 40)

→ 1/30 - 1/40 = 1/v

→ (4 - 3)/120 = 1/v

→ v = 120

Hence image is formed beyond Center of Curvature.