Question

What is meant by power of a lens? You have three lenses L1, L2 and L3 of powers +10D, +5D and -10D respectively.

State the nature and focal length of each lens. Explain which of the three lenses will form a virtual and magnified

image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer.​

Answer 1

Power of a lens is the degree of convergence of divergence of light rays achieved by a lens.

Lens L1 : f1 =

=

= +10 cm ; Convex lens

Lens L2 : f2 =

=

= +20 cm ; Convex lens

Lens L3 : f3 =

=

= -10 cm ; Concave lens

Lens L2 will form a virtual and magnified image of an object placed at 15 cm from the

convex lens because concave lens can never form virtual and magnified image of an object

and convex lens form such image only when the object is placed between the optical centre

and principle focus of the convex lens.

For diagram refer NCERT Page 181 figure 10.16 (f)