A lens forms a virtual image 4 cm away from it when an object is placed 10 cm away from it. The lens is a .....lens of focal length .....

Question

A lens forms a virtual image 4 cm away from it when an object is placed 10 cm away from it. The lens is a .....lens of focal length ..... (A) concave, 6.67 cm (B) concave, 2.86 cm (C) convex, 2.86 cm (D) may be concave or convex, 6.67 cm.

Tardigrade Admin · Accepted Answer

(1/f)=(1/v)=(1/u) (1/f)=(1/-4)-(1/-10) or (1/f)=(1/10)-(1/4) or (1/f)=(2-5/20)=-(3/20) or f=-(20/3) cm =-6.67 cm The negative sign indicates that the lens is concave.

Q. A lens forms a virtual image $4 c m$ away from it when an object is placed $10 c m$ away from it. The lens is a .....lens of focal length .....

concave, 6.67 cm

concave, 2.86 cm

convex, 2.86 cm

may be concave or convex, 6.67 cm.

$\frac{1}{f} = \frac{1}{v} = \frac{1}{u}$
$\frac{1}{f} = \frac{1}{- 4} - \frac{1}{- 10}$
or $\frac{1}{f} = \frac{1}{10} - \frac{1}{4}$ or $\frac{1}{f} = \frac{2 - 5}{20} = - \frac{3}{20}$
or $f = - \frac{20}{3} c m = - 6.67 c m$
The negative sign indicates that the lens is concave.

Q. A lens forms a virtual image 4cm away from it when an object is placed 10cm away from it. The lens is a .....lens of focal length .....