Question

The magnifying power of a telescope is 9. When it is adjusted for parallel rays, the distance between the objective and the eye piece is found to be 20 cm. The focal length of lenses are(a) 18 cm, 2 cm(b) 11 cm, 9 cm(c) 10 cm, 10 cm(d) 15 cm, 5 cm

Answer 1

(a) 18 cm, 2 cm is the answer

Answer 2

Answer:

A) 18cm and 2cm

Explanation:

Magnification power of telescope M = 9  (Given)

Distance between the objective and the eye piece = 20 cm (Given)

Thus, magnification will be calculated as -

M = fo/fe

9 = fo/fe

fo = 9fe

Also, fo+fe = 20  ( This is because the final image is at infinity)

10fe = 20

fe = 20/10

fe = 2

fo = 2×9

= 18cm

Thus, the focal length of lenses are 18cm and 2cm