Question

A biconvex lens (R₁ = R₂ = 20 cm) has focal length equal to focal length of concave mirror. The radius of curvature of concave mirror is ______.

R.I. of glass lens= 1.5.

Options

• -20 cm

• 40 cm

• 20 cm

• -40 cm

Answer 1

A biconvex lens (R₁ = R₂ = 20 cm) has focal length equal to focal length of concave mirror. The radius of curvature of concave mirror is -40 cm.

Explanation:

`1/"f"=("n"-1)(1/"R"_1-1/"R"_2)`

n = 1.5, R₁ = 20 cm, R₂ = -20 cm

∴ f = 20 cm

for a concave mirror, R = 2f and it is negative.

∴ R = -40 cm