The parabola has an electromagnetic signal reflection property. Four signals are shown in green and blue. These signals are shown with arrows on both ends to indicate the focus either collects the signals (coming in) or the focus generates the signals and they leave in parallel from the parabola. The inside of the parabola can be a mirror (for light) or another material (for non visible electromagnetic waves.) As light enters parallel to axis of symmetry it will strike the parabola and reflect toward the focus. You can see heavy black line segments drawn on the parabola on the lines tangent to the parabola at the points of incidence. Two angles are formed between each of these segments and the light striking and bouncing off; each pair of angles are equal and depend upon the location the light hits the parabola. Imagine the focus is a light bulb and the parabola a mirror. The light bulb emits light in all directions. All the light that strikes the parabola will leave parallel to the axis of symmetry. Spot lights make use of this property. Of course a a parabolic mirror is 3-dimensional. Imagine rotating the parabola about its axis of symmetry and you will get a shape you'll recognize as the headlight of your car.
Light emitted from the focus leaves the parabolic mirror in parallel paths, shown below. Headlights, spotlights, etc., have the shape of a parabola to increase the intensity of the light and direct the light. The ellipse has similar reflective properties. Below you see three lines, blue, green and red. These lines start from either focus and 'bounce' off the ellipse toward the other focus. As with the parabola, the angles between each signal and its tangent line segment (dark black) are equal.
The parabola also amplifies any signal entering it directing it to the focus. Satellite dishes use this property as do dishes used at astronomical observatories.
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