This applet shows Huygens principle in action.[br]In the animation, (blue) waves are generated in the focus of a parabola and on reflecting on it (green) secondary wavelets are produced. The envelopes of the secondary wavelets are, as expected, plain waves.[br]You can adjust the shape of the parabola by acting on its upmost point, and choose wavelength, number of waves and number of wavelets by acting on the sliders.