Question

A convex lens of focal length 0.2 m and made of glass 0.5 is immersed in water find the change in focal length of the lens

Answer 1

Answer:Formula for the focal length of the lens is :

1/f = (ng​-nm)(1/R1-1/R2)

f= foal length of the lens

ng= refractive index of the glass=1.5

n​m= refrctive index of the medium

R1&R2 are the radius of curvature near and far from the sorce respectively.

in air , refractive index of the air is 1.

Hence, the focal length in air is given by :-

1/fa = (1.5-1)(1/R1-1/R2) ------(1)

in water, refractive index of the water is 1.33.

Hence, the focal length in water is given by :​

1/fw = (1.5-1.33)(1/R1-1/R2) -----(2)

divide equation no. (1) and (2), we get

fw​/fa = (1.5-1)/(1.5-1.33)

= .5/.17

= 2.94

Given :- fa = 20cm

fw = 20*2.94

=58.80cm

Hence, focal length of the lens in the water in 58.80cm.