Question

Find the power of a convex lens which forms a real and inverted image of magnification -1 of an object placed at a distance 20 cm from its optical centre.

Answer 1

→ m = v/u

→ - 1 = v/( - 20)

→ 20 = v

[Additional info = Image is formed at Center of Curvature]

Lens Formula

→ 1/f = 1/v - 1/u

→ 1/f = 1/(20) - 1/(- 20)

→ 1/f = 1/20 + 1/20

→ 1/f = 1/10

→ f = 10 cm

Power = 1/f

[Note : Unit of “f” in power should be meter]

→ 10 cm = 0.1 m

→ Power = 1/(0.1)

→ Power = + 10

Hence power of convex lens is + 10.