Question

Size of the image of an object by a concave lens of focal length
20 cm is observed to be reduced to a rd of its size. Find the distance
of the object from the lens.
​

Answer 1

Answer: The distance of the object is 40 cm from the lens.

Explanation:

Given that,

Focal length f = -20 cm

Magnification m =\dfrac{1}{3}

We know that,

The magnification is

m = \dfrac{v}{u}

\dfrac{1}{3}=\dfrac{v}{u}

u = 3v......(I)

Using lens formula

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

\dfrac{1}{-20}=\dfrac{1}{v}-\dfrac{1}{3v}

v = \dfrac{-40}{3}

Put the value of v in equation (I)

u = -40\ cm

Hence, The distance of the object is 40 cm from the lens.