The applet below allows you to investigate the reflective properties of a parabola. Move the mouse on the screen below, when it is "inside" the parabola a set of crosshairs will appear. When the mouse is clicked a light ray parallel to the axis of symmetry of the the parabola through the point is drawn. When the ray hits the parabolic mirror it is reflected, and you can see the path of the reflected ray. If you drag the mouse while it is clicked, you can move the location of your light ray. Up to 5 rays can be drawn at one time. The rays can be erased by clicking the button. The width of the parabola can be changed by moving the slider. Notice how the location of the focus changes as well.