Question

focal length of biconvex glass lens having radii of curvatures 50cm and 40cm has focal length 44cm when kept in air medium . calculate focal length of same lens when air is replaced by water of R.I 1.33

Answer 1

f = R/2

For a thin convex lens we have the relation

1/f = (μ - 1)(1/R1 - 1/R2)

f - focal length = R/2
μ - Refractive index
R1, R2 the radii of curvature of both the surfaces
Here R1 = R and R2 = -R

Therefore 1/f = (μ - 1)(1/R - 1/(-R))
2/R = (μ - 1)(2/R)

(μ - 1) = 1

⇒ μ = 2

Answer 2

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