Question

What is meant by power of a lens?
You have two lenses A and B of focal lengths +10 cm and -10 cm respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens? Draw a ray diagram to justify your answer.

Answer 1

Power of a lens is defined as the ability of less to converge or to diverge a beam of light. It is also defined as reciprocal of focal length in metres.
Let $f_A$ be focal length of lens A and $P_A$ be the power of lens A.
For lens A:
$f_A$ = + 10 cm = 0.1 m
Since the focal length is positive, it is a convex lens.
Now, $P_A=\frac{1}{f_A}=\frac{1}{0.1}=+10$D.
For lens B : $f_B$ = -10 cm = - 0.1 m
Let $f_B$ be focal length of lens A and $P_B$ be the power of lens B.
Since the focal length is negative, it is a concave lens.
$P_B=\frac{1}{f_B}=\frac{1}{-0.1}=-10$D.
In case of a convex lens, image is virtual and magnified if object is placed between focus and optical centre.