Question

Sunlight reflects from a concave piece of broken glass, converging to a point 34 cm from the glass. what is the radius of curvature of the glass?

Answer 1

The rays of light coming from the Sun are parallel to each other, so when they are reflected by the concave piece of glass (which acts as a concave mirror) they converge into the focus of the mirror, which is
$f=34 cm$
The radius of curvature of a concave mirror is twice its focal length, so in this case it is:
$r=2f = 2 \cdot 34 cm=78 cm$