Question

a plano convex lens of focal length 120 cm has radius of curvature of its convex surface 60 cm.calculate focal length in water. ri is 4÷3​

Answer 1

Answer:

(1/f) = (n - 1)( 1R1 - 1R2) put R2 →∞ for plane surfaceand R1 = 10cm with f = 30cm we get refractive index n = 4/3 = 1.33

Answer 2

The focal length of plano-convex lens in water is 180 cm.

Explanation:

The focal length of plano-convex lens is given by the formula:

1/f = (n - 1)(1/R₁ - 1/R₂)

Where,

f = Focal length of lens in water

n = Refractive index of water = 4/3

R₁ = Radius of curvature of convex surface = 60 cm

R₂ = Radius of curvature of plane surface = ∞

On substituting the values, we get,

1/f = (4/3 - 1)(1/60 - 1/∞)

1/f = ((4 - 3)/3)(1/60 - 0)

1/f = 1/3 × 1/60

1/f = 1/180

∴ f = 180 cm