Question

The effective focal length of Ramsden’s eyepiece is 3cm. The focal length of a single lens is ___________

(a) 3cm

(b) 4cm

(c) 5cm

(d) 6cm

Answer 1

Correct option is (b) 4cm

For explanation I would say: Here, F = 3 cm, f =? d =?

In Ramsden’s eyepiece, F = f and d = 2f/3

Therefore, applying the formula

(frac{1}{F}=frac{1}{F}+frac{1}{f}-frac{d}{Ff})

F = 3f/4

f = 4F/3

f = 4 X 3 /3

f = 4 cm.