Question

The focal length of a glass convex lens is is 1.5 centimetre in air calculate the focal length why it is immersed in water

Answer 1

Answer:

When a lens is immersed in water , the value of refractive index decrease . Hence , its focal length increases .

Here, it is a convex lens .

Refractive index of lens , ᵃμg = 1.5

Focal length of lens in air , fa  =  8  cm

Refractive index of water , ᵃμw = 1.33

The radius of curvatures are R1 and R2 .

For the lens in air -

1/fa  = (ᵃμg - 1) * [1/R1 - 1/R2]

=>  1/8  = (1.5 - 1) * [1/R1 - 1/R2]

=>  [1/R1 - 1/R2]  =  1/4

When lens is immersed in water

1/fw  = [(ᵃμg/ᵃμw) - 1] * [1/R1 - 1/R2]

where, fw  =  focal length of lens in water

=>  1/fw =  [(1.5/1.33) - 1] * (1/4)

=>  1/fw  = (1/8) * (1/4)

             =  1/32

=>  fw  =  32   cm

Thus, focal length of lens when immersed in water =  32  cm