A parabola is the set of all points that are the same distance from a fixed point (the focus) and a fixed line (the directrix). Move the sliders to change the focus's position and the point on the parabola and confirm this definition, then explore the reflection property used in satellite dishes and flashlights.
p is the distance from the vertex to the focus. Move the point on the parabola with the slider, and check whether its distance to the focus (violet dashed line) and its distance to the directrix (coral dashed line) are always equal.
Distance to the focus (p)2
Position of the point on the parabola3
Light (dashed arrows) coming in parallel to the parabola's axis always reflects toward the focus once it hits the parabola's surface. That's why satellite dishes and flashlight reflectors are shaped like a parabola (paraboloid).