A concave lens of focal length 20 cm produces an image half the size of the real object. The distance of the real object is

Question

A concave lens of focal length 20 cm produces an image half the size of the real object. The distance of the real object is (A) 20 cm (B) 30 cm (C) 10 cm (D) 60 cm

Tardigrade Admin · Accepted Answer

From lens formula (1/f)= (1/v)= (1/u) Given, v = (u/2) and f=20 cm ⇒ (1/20)= (1/u/2)= (1/u)= (1/u) ⇒ u = 20 cm

Q. A concave lens of focal length 20 cm produces an image half the size of the real object. The distance of the real object is

20 cm

30 cm

10 cm

60 cm

40 cm

From lens formula
$\frac{1}{f} = \frac{1}{v} = \frac{1}{u}$
Given, $v = \frac{u}{2}$ and $f = 20 c m$
$\Rightarrow \frac{1}{20} = \frac{1}{u /2} = \frac{1}{u} = \frac{1}{u}$
$\Rightarrow u = 20 c m$