Question

A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 40 cm. Calculate the :
(a) Power of each lens
(b) Power of resulted lens
(c) Nature of resulted lens
(d) Focal length of the resulted lens
[5 MARKS]

Answer 1

part a: 2 Marks
part b,c and d: 1 Mark each

(a) $\because$ Power of lens, $P=\frac{100}{f\left(incm\right)}$
$\therefore$ Power of convex lens, $P_1=\frac{100}{20}=+5D$
Power of concave lens, $P_2=\frac{-100}{40}$
$=-2.5D$
(b) Power of the resulted lens,
$P=P_1+P_2=+5D+(-2.5D)=+2.5D$
(c) As the power of resulted lens is positive therefore, it behaves like a convex lens.
(d) Focal length of resulted lens : f(in cm)
$=\frac{100}{P}=\frac{100}{2.5}=40cm$