Question

The focal length of the combination of lenses of focal lengths-70 cm and 150 cm is a) 131.25 cm b)-131.25 cm c) 132.15 cm d) 132.15cm​

Answer 1

Given:

Combination of lenses of focal lengths-70 cm and 150 cm.

To find:

Net focal length of combination?

Calculation:

We know that combination of lens follows the rule:

$\boxed{P = P1 + P2}$

• "P" represents the power of the lens.

$\implies \: \dfrac{1}{f} = \dfrac{1}{f1} + \dfrac{1}{f2}$

$\implies \: \dfrac{1}{f} = \dfrac{1}{( - 70)} + \dfrac{1}{150}$

$\implies \: \dfrac{1}{f} = - \dfrac{1}{70} + \dfrac{1}{150}$

$\implies \: \dfrac{1}{f} = \dfrac{ - 15 + 7}{1050}$

$\implies \: \dfrac{1}{f} = \dfrac{ - 8}{1050}$

$\implies \:f = \dfrac{ - 1050}{8}$

$\implies \:f = - 131.25 \: cm$

So, net focal length is -131.25 cm.