Question

A convergent beam of light passes through a diverging lens of focal length 0.2 m and comes to focus 0.3 m behind the lens. The position of the point at which the beam would converge in the absence of the lens is :

• A. 0.12 m
• B. 0.6 m
• C. 0.3 m
• D. 0.15 m

Answer 1

The correct option is A 0.12 m

Here, $f=-0.2m,v=+0.3m$

from lens equation

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

$\frac{1}{u}=\frac{1}{v}-\frac{1}{f}=\frac{1}{0.3}-\frac{1}{0.2}=\frac{25}{3}$

$u=\frac{3}{25}=0.12m$

Therefore, in the absence of lens, the beam would coverage at a distance of $0.12m$ to the right of the concave lens.