Question

For a thin convex lens when the heights of the object is double than its image, its object distance is equal to _________ focal length of a lens is f
(A) f (B) 2f (C) 3f (D) 4f.​

Answer 1

Answer:

(C) 3f

Explanation:

please find attached my explanation for this question

Answer 2

Given that,

When the heights of the object is double than its image.

$h_{o}=2h_{i}$

We need to calculate the object distance

Using formula of magnification

$m=\dfrac{h_{i}}{h_{o}}=-\dfrac{v}{u}$

Put the value into the formula

$\dfrac{h_{i}}{2h_{i}}=-\dfrac{v}{u}$

$-\dfrac{1}{2}=\dfrac{v}{u}$

$v=-\dfrac{u}{2}$

We  need to calculate the focal length

Using formula of lens

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

Put the value into the formula

$\dfrac{1}{f}=\dfrac{-2}{u}-\dfrac{1}{u}$

$\dfrac{1}{f}= \dfrac{-3}{u}$

$u=-3f$

Hence, The object distance is equal to the 3f.

(C) is correct option.