The sweep rate of the tail of a comet can exceed the speed of light. Explain this phenomenon and show that there is no contradiction with relativity.

The largest part of the comet is long tail. It emerges from a cloud of gas called the coma. A major part of comet’s light is created by fluorescence. A photon of ultraviolet and thus energetic radiation from the Sun lifts electrons in the atoms of the comet’s gas molecules to an upper, excited level in a single leap. The electron then returns to its original level in two or more steps, emitting photon each time it drops. The combined energy of these photons must equal that of the absorbed ultraviolet photon to conserve energy. Thus, the energy of each emitted photon must be less than that of the original ultraviolet one. That smaller energy then gives them a longer wavelength, which we can see with our eyes. Thus, fluorescence creates the soft glow of the comet’s light image. Since the image of light can move faster than the speed of light, therefore the sweep rate of the tail of a comet can exceed the speed of light and there is no contradiction with relativity.