Light reflects off a curved surface by forming [color=#a61c00][i]equal angles[/i][/color] ([color=#85200c][i]incidence [/i][/color]and [i][color=#85200c]reflection[/color][/i]) [color=#85200c][i]with the tangent[/i][/color] at the point it falls on the surface.[br][br]If we consider a parabola as a reflecting surface, when a [color=#1155cc][i]ray of light emitted from the focus[/i][/color] meets a [color=#1155cc][i]point on the parabola[/i][/color], it will [color=#1155cc][i]reflect[/i] [/color]off in a [color=#1155cc][i]straight line parallel to the axis of the parabola[/i][/color].[br][br]The converse also holds, and it's the principle used - for example - to design satellite dishes: any [i][color=#0b5394]ray parallel to the axis [/color][/i]of the parabola will [i][color=#0b5394]reflect [/color][/i]off the parabola and[i][color=#0b5394] pass through the focus[/color][/i]. [br][br][br]Start the animation in the applet below to view this optical property of parabolas. [br]If you [b]Show details[/b], you will see the geometry behind this property: move the point of incidence to verify that the property holds for all points of incidence on the parabola.