2620 Sunlight reflects from a concave piece of broken glass

26.20 Sunlight reflects from a concave piece of broken glass, converging to a point 13 cm from the glass.

What is the radius of curvature of the glass?

Express your answer using two significant figures.

Solution

Parallel light into a concave piece of glass focuses on the focal point
The radius of curvature is twice the focal length

So R = 2*f = 2*13cm = 26 cm