Question

A thin converging lens and a thin diverging lens of each focal length 10 cm are placed coaxially 5cm apart. What will be the focal length of the combination?

(a) +10cm

(b) -10cm

(c) +20cm

(d) -20cm

Answer 1

Right answer is (c) +20cm

Explanation: We know, f1=10 cm, f2 = -10 cm, d = 5 cm

(frac{1}{f}=frac{1}{f_1}+frac{1}{f_2}-frac{d}{f_1f_2})

= 1/10 – 1/10 + 5/100

f = + 20 cm.