Any ray parallel to the axis of symmetry of a parabola will reflect to the focus regardless of its position. The [math]xvar[/math] slider changes the position of the source while keeping the ray parallel to the axis of symmetry. The sliders [math]xoffset[/math] and [math]yoffset[/math] offers a way to move the source such that the ray direction is not parallel to the axis of symmetry anymore.[br][br]Play around with these sliders and observe the behavior of the reflected ray.