Question

11. How will be the change in focal length of a glass convex lens when it is immersed in
benzene solution. The refractive indices of water, glass and benzene are 1.33. 1.45.
1.50 respectively​

Answer 1

when convex lens is immersed in benzene solution, it behaves as diverging lens.

using formula, 1/f = (μ - 1)[1/R1 - 1/R2]

for convex lens focal length is positive.

so, 1/f = (μ - 1) [1/R1 - 1/R2] > 0

⇒[1/R1 - 1/R2] > 0 .......(1)

when convex lens is immersed in water,

1/f_w = (μ_g/μ_w - 1) [1/R1 - 1/R2]

= (1.45/1.33 - 1) [1/R1 - 1/R2 ] > 0

so, nature of convex doesn't change just change its focal length. focal length of convex lens increases when it is immersed in water.

when convex lens is immersed in benzene solution .

1/f_b = (μ_g/μ_b - 1) [1/R1 - 1/R2]

= (1.45/1.50 - 1)[1/R1 - 1/R2]

as (1.45/1.50 - 1) is negative and from equation (1), [1/R1 - 1/R2] is positive

so, 1/f_b < 0 , f_b < 0

hence, convex lens behaves as diverging lens.

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Answer 2

$\huge\bold\purple{Answer:-}$

using formula, 1/f = (μ - 1)[1/R1 - 1/R2]

for convex lens focal length is positive.

so, 1/f = (μ - 1) [1/R1 - 1/R2] > 0

⇒[1/R1 - 1/R2] > 0 .......(1)

when convex lens is immersed in water,

1/f_w = (μ_g/μ_w - 1) [1/R1 - 1/R2]

= (1.45/1.33 - 1) [1/R1 - 1/R2 ] > 0

so, nature of convex doesn't change just change its focal length. focal length of convex lens increases when it is immersed in water.

when convex lens is immersed in benzene solution .

1/f_b = (μ_g/μ_b - 1) [1/R1 - 1/R2]

= (1.45/1.50 - 1)[1/R1 - 1/R2]

as (1.45/1.50 - 1) is negative and from equation (1), [1/R1 - 1/R2] is positive

so, 1/f_b < 0 , f_b < 0

hence, convex lens behaves as diverging lens.