Question

Two lenses of focal length 8 cm and 6 cm are placed a certain distance apart. If they form an achromatic combination, the separation between them is ___________

(a) 5cm

(b) 6cm

(c) 7cm

(d) 8cm

Answer 1

Correct choice is (c) 7cm

To elaborate: For an achromatic combination, the separation x is given by,

x = (f1 + f2)/2

As we know, f1 = 8cm and f2 = 6cm

x = 14/2

x = 7 cm.