Question

A convex lens of focal length 10 cm and a concave
lens of focal length 20cm are placed in contact
with each other.
a) what is the power of this combination?
b) What is the focal length of this combination ?​

Answer 1

Answer:

Total focal length = 6.66 cm

Total power = 30 D

Explanation:

a) If the lenses are in contact,  total focal length is given by the formulae :

$\frac{1}{f} = \frac{1}{f_{1} } + \frac{1}{f_{2} }$

   = 1/10 + 1/20

   = 3/20

f = 20/3 cm = 6.66 cm

b) Total power is given by :

$P = P_{1} + P_{2}$

    = 10 + 20 = 30 D