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

Answer:

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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

Solution :-

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

➙ $\bf{f'\:=\:-160\:cm}$

➙ f′ =−160cm

Hence,

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

Here we can neglecte the sign.