Question

An astronomical telescope uses two lenses of powers 10 dioptres and 1 dioptre. If the final image of a distant object is formed at infinity, calculate the length of the telescope

Answer 1

If f₀ and f_e denoted the focal lengths of the objective and the eye lens of the telescope, then
f₀ = `100/10` m = 10 cm

and  Since the final image is formed at infinity, the telescope is in normal adjustment. Further, we know that the distance between the objective lens and the eye lens in normal adjustment.
= f₀ + f_e
= 10 + 100 = 110 cm