Question

A glass convex lens of refractive index 1.5 has focal length 8 cm when in air what would be focal length of lens when immersed in water of refractive index 1.33​

Answer 1

Given :

Refractive index of lens= 1.5

focal length in air=8cm

from lens makers formula :

1/f= (n-1)[1/R1-1/R2]

1/8= (1.5-1)[1/R1-1/R2]

1/8= 0.5[1/R1-1/R2]

1x10/8 x 5=[1/R1-1/R2]

[1/R1-1/R2]=2/8

[1/R1-1/R2]=1/4-------------------1

when lens in immersed in water of refractive index = 1.33

1/fw=( n1/n2-1)[[1/R1-1/R2]

1/fw=( 1.5/1.33-1)[1/R1-1/R2]

1/fw= 1/8 x 1/4

fw=32

fw=32cm

∴ Focal length of lens in water =32cm