Question

If the the focal length of convex lens in convex lens in air is 40cm when it is immersing in water then the focal length will be(a)20 cm (b)160 cm (c) 80cm (d) 40cm ​

Answer 1

Given :-

Focal length in air = f = 40 cm

Let the focal length in medium = f'

Refractive index of air = μ = 1

Refractive index of water = μ' = 4/3

f ∞ 1/μ

f'/f= μ/(μ/μ'-1)

f'/40 = 1/(1/4/3 -1)

f' = (40 × 4)/3-4

$\bf{f'\:=\:-160\:cm}$

Hence,

The focal length in medium = f' = -160 cm

Here we can neglecte the sign.

Answer 2

Explanation:

given :

• focal length of convex = 40 cm

• when it is immersing in water then the focal length will be

to find :

• the focal length will be

solution :

• focal = F

• index refractive = 1

• index refractive = 4/3

• focal = 1/μ

• focal= µ/(μ/µ-1)

• focal/40 = 1/(1/4/3-1)

• focal= (40 x 4)/3-4

• focal = -160

focal = -160