Question

What is the focal length of double concave lens kept in air with two sperical substances if radii?

Answer 1

Given,

The radius of the first lens = 30 cm.

The radius of the second lens = 60 cm.

The refractive index = 1.5

i.e. R1 = 30cm

R2 = 60 cm

n = 1.5

To find out,

The focal length of double concave lens kept in air.

ANSWER:

Formula:

$\frac{1}{f} = (n - 1)( \frac{1}{R1} - \frac{1}{R2} )$

$\frac{1}{f} = (1.5 - 1)( \frac{1}{ - 30} - \frac{1}{60} )$

$\frac{1}{f} = (0.5)( \frac{ - 2 - 1}{60} )$

$\frac{1}{f} = (0.5)( \frac{ - 3}{60} )$

$\frac{1}{f} = \frac{5}{10} \times \frac{ - 3}{60}$

$\frac{1}{f} = \frac{1}{10} \times \frac{ - 1}{4}$

$\frac{1}{f} = \frac{ - 1}{40}$

$f = - 40cm$

Therefore the focal length of double concave mirror kept in air is -40cm.