Question

Lenses of powers $3D$ and $-5D$ are combined to from a composite lens. An object is placed at a distance of $50cm$ from this lens. Calculate the position of its image.

• A. $-10cm$
• B. $+10cm$
• C. $-25cm$
• D. $+25cm$

Answer 1

The correct option is C $-25cm$

Given,

Power $P_1=+3Dand{P}_2=-5D$

Position of object $u=-50cm$

Power gets added

$P=P_1+P_2$

$P=3-5=-2$

$f=\frac{1}{P}=\frac{1}{-2}=-50cm$

Apply lens formula

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

$v=\frac{uf}{f+u}=\frac{(-50)\times (-50)}{(-50)+(-50)}=-25cm$

Position of image is $v=-25cm$