The parabolic curve has a special importance in optics. Consider for example a very weak radio signal coming from a satellite in orbit. If you use just a regular radio receiver, the signal will be so weak as to be indistinguishable from the background noise. However, if you use a parabolic satellite dish to collect the power from a large surface area and focus it on the receiver, then you will be able to detect the signal. This works because of the parabolic shape of the satellite dish: all radio wave coming in from far away will get reflected towards the same point—the focal point of the parabola. Thus, if you put your receiver at the focal point, it will have the signal power from the whole dish redirected right to it.

Depending on the shape of the parabola (which way it curves and how strong the curvature is) the focal point or focus will be at a different place. In the next two sections, we will study parabolic mirrors and lenses. We will use the “horizontal rays get reflected towards the focus”-fact to draw optics diagrams and calculate where images will be formed.