Question

A student placed an object at a distance of 60 cm in front of a convex lens and found that its real and inverted image is formed 30 cm behind the lens. The power of the lens is​

Answer 1

Answer:

1/f= 1/u + 1/v ( without sign convensation)

F= focal length

U= object distance

V= image distance

Power= 1/f ( in meters)

1/f = 1/60 +1/30

=. 1/60+ 2/60

1/f = 3/60

F= 60/3 = 20 cm

Power= 1/f = 1/ 0.2

= 0.05 Diopters

Power= + 0.05 D

(Convex lenses have positive power values. Concave lenses have negative power values)

Explanation:

Answer 2

Given:

A student placed an object at a distance of 60 cm in front of a convex lens and found that its real and inverted image is formed 30 cm behind lens.

To find:

Power of lens ?

Calculation:

Applying Len's Formula:

$\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$\implies \dfrac{1}{f} = \dfrac{1}{30} - \dfrac{1}{ (- 60)}$

$\implies \dfrac{1}{f} = \dfrac{1}{30} + \dfrac{1}{ 60}$

$\implies \dfrac{1}{f} = \dfrac{2 + 1}{ 60}$

$\implies \dfrac{1}{f} = \dfrac{3}{ 60}$

$\implies f= 20 \: cm$

• Now, power is reciprocal of focal length (in metres).

$P = \dfrac{1}{f \: (in \: metres)}$

$\implies P = \dfrac{1}{0.2}$

$\implies P = 5 \: D$

So, power of lens is 5 DIOPTRE.