Question

A thin converging lens of focal length 12 cm is kept in contact with a thin diverging lens of focal length 18 cm. Calculate the effective/equivalent focal length of the combination.

Answer 1

Focal length of converging lens is positive, hence

f₁ = + 12 cm

Focal length of diverging lens is negative, hence

f₂ = - 18 cm

Equivalent focal length of the combination is given by,

`1/"f" = 1/"f"_1 + 1/(- "f"_2)`

`1/"f" = 1/12 - 1/18`

`1/"f" = 1/36`

⇒ f = 36 cm