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
[tex]f=34 cm[/tex]
The radius of curvature of a concave mirror is twice its focal length, so in this case it is:
[tex]r=2f = 2 \cdot 34 cm=78 cm[/tex]
