Question

An achromatic convergent doublet of two lenses in contact has a power of + 2D. The convex lens has power + 5D. What is the ratio of dispersive powers of convergent and divergent lenses?(a) 2 : 5(b) 3 : 5(c) 5 : 2(d) 5 : 3

Answer 1

the correct answer is (a) option.

Answer 2

Answer:

B) 3:5

Explanation:

Contact lens power P = 2D (Given)

Convex lens power P1 = 5D (Given)

Thus power -

P2 = P – P1 = 2 – 5 = –3D

Therefore the focal length of the divergent lens f2 will be = 100/3cm

and the focal length of the convergent lens f1 will be = 100/5 = 20cm

ω1/ω2 = -f1/f2 = -P2/P1 = 20/100/3  = 3/5

Thus, the ratio of dispersive powers of convergent and divergent lenses is 3:5