Question

What is the ratio of the focal lengths of the two Plano-convex lenses in Huygens’s Eyepiece?

(a) 2:1

(b) 3:1

(c) 3:2

(d) 4:3

Answer 1

Correct answer is (b) 3:1

To elaborate: The focal lengths of the two Plano-convex lenses in Huygens’s Eyepiece are in the ration 3:1(3f and f) and the distance between them is equal to 2f. The focal length and positions of the two lenses are such that the eyepiece is achromatic and free from spherical aberration.