Question

The radius of curvature of a thin plano-convex lens is 10 cm (of curved surface) and the refractive index is 1.5. If the plane surface is silvered, then it behaves like a concave mirror of focal length(a) 10 cm(b) 15 cm(c) 20 cm(d) 5 cm

Answer 1

Hola User

c}20 cm

Thanks

Answer 2

Answer:

C) 20cm

Explanation:

Radius = 10cm (Given)

Refractive index = 1.5 (Given)

The silvered plano convex lens behaves as a concave mirror, whose focal length is denoted as -

1/F = 2/f1+1/fm

If the plane surface is silvered -

fm = R2/2 = ∞/2 = ∞

1/f1 = (u-1)(1/R1-1/R2)

1/f1 = u-1/R

1/F = 2(u-1)/R + 1/∞ = F = R/2(u-1)

Where r = 20cm and u = 1.5

Thus f = 20cm

Therefore, f the plane surface is silvered, then it behaves like a concave mirror of focal length 20cm