Question

the refractive index of the material of a concave lens is 1.5 and radii of curvature are 40 cm and 60 cm respectively. find the focal length of the lens when it is kept in air

Answer 1

Answer:

focal length will decrease

Using lens makers formula

(1.5-1) (1/-40-1-/60)

0.5(-20/2400)

0.5(1/-120)

1/f=-1/240

f=-240

Answer 2

The focal length of the mirror is 240 cm

Explanation:

Given:

The refractive index of concave lens $\mu=1.5$

The radius of curvature

$R_1=40$ cm

$R_2=60$ cm

To find out:

The focal length of the lens in air

Solution:

We know that the relation between the given quantities and focal length $f$ is

Since the lens is concave, $R_1$ will be left facing hence negative and $R_2$ will be right facing hence positive

$\frac{1}{f}=(\mu-1)(\frac{1}{R_1}-\frac{1}{R_2})$

$\implies \frac{1}{f}=(1.5-1)[\frac{1}{(-40)}-\frac{1}{(-60)}]$

$\implies \frac{1}{f}=(1.5-1)[-\frac{1}{40}+\frac{1}{60}]$

$\implies \frac{1}{f}=0.5\times(-\frac{1}{120})$

$\implies \frac{1}{f}=-\frac{1}{240}$

$\implies f=-240$ cm

Hope this answer is helpful.

Know More:

Q: A double concave lens with the refractive index 1.5 is kept in the air . Its two spherical surfaces have radii R1 = 20 cm and R2 = 60 cm . Find the focal length of the lens . Write the characteristics of the lens.

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