## Question

The radius of curvature of the convex surface of a thin plano-convex lens is 20 cm. Refractive index of the medium of the lens is 1.5. Calculate the focal length of the lens.

### Solution

40 cm

For the focal length *f* of a lens in air, we have

Where *n* is the refractive index of lens material (glass) with respect to air, and R_{1} and R_{2} are radii of curvature of the lens surfaces.

Here *n* = 1.5, *R*_{1} = +20 cm and *R*_{2} = ∞ (plane).

or *f* = **40 cm**.

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