Parabolic Reflectors
If a reflector is axially symmetrical and shaped so its cross-section is a parabola, it has the property of bringing light that has come from a very distant source such as the sun, so the rays of light are effectively parallel, to a point focus. If the axis of symmetry is aimed at the sun, any object that is located at the focus receives highly concentrated sunlight, and therefore becomes very hot. This is the basis for the use of this kind of reflector for solar cooking.
Read more about this topic: Solar Cooker