Q. 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
Explanation
(a) $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\frac{1}{f} = \frac{1}{- 4} - \frac{1}{- 10}$
Or $\ \frac{1}{f} = \frac{1}{10} - \frac{1}{4}$
Or $\frac{1}{f} = \frac{2 - 5}{20} = - \frac{3}{20}$
Or $\ f = - \frac{20}{3}cm = - 6.67\ cm$
The negative sign indicates that the lens is concave
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.