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.
Answers
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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