Question

A small telescope has an objective lens of focal length 150 cm and eye piece of focal length 5 cm if this telescope is used to view a 100 m high tower 3 km away, find the height of the image of the tower formed by the objective lens

Answer 1

Height of image will be 5 cm.

Solution:

Height of tower = h'= 100m

Distance from tower = 3km= 3000m

Focal length of objective lens = f'= 150 cm

height of image = h= ?

angle made by tower =  α = 100/(3 x 1000) = 1/30 rad

h/f' = α

putting value of α and f'

h/150 = 1/30

h = 1/30 x 150

h = 5 cm.

I hope it work