Question

.An object placed 10 cm in front of a lens has an image 20 cm behind the lens. What is the power of the lens

Answer 1

Given :

Object distance= u= - 10cm [ assumed as convex lens ]

Image distance=v=20cm

focal length = f= 1/v-1/u

= 1/20+1/10

1/f= 3/20

Power = P= 100/f  ( in cm)

P= 100x3/20

=300/20

=+15 D

∴ power of lens is +15D.

Answer 2

Answer:

The power of the lens is 15 D.

Explanation:

Given that,

Distance of the object u = -10 cm

Distance of the image v = 20 cm

Using lens's formula

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

Put the value into the formula

$\dfrac{1}{f}=\dfrac{1}{20}-\dfrac{1}{-10}$

$\dfrac{1}{f}=\dfrac{3}{20}$

$f =\dfrac{20}{3}\ cm$

The power of the lens is the reciprocal of its the focal length.

The power of the lens is defined as:

$P =\dfrac{100}{f}$

$P = \dfrac{100\times3}{20}$

$P = 15\ D$

Hence, The power of the lens is 15 D.