Question

The objective of a telescope consists of two lenses kept in contact. One lens has an optical power of +2.0 D whereas the other lens has an optical power of −1.5 D. Calculate focal length of the objective.

Answer 1

Given: P₁ = +20. D

P₂ = −1.5 D

∴ P = P₁ + P₂

= 2.0 D + (−1.5 D)

= 0.5 D

We know that `1/f` = P

⇒ `1/f` = 0.5

⇒ f = `1/0.5` m

= 2 m