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Originally Posted by jnb  Well as in any material rather than a vacuum there will be the phenomenon of dispersion (the same phenomenon which gives rise to spectra in a prism). Then the dispersion is a function of wavelength and so the speed of light in a medium is a function of its frequency and thus energy. So by that measure as 'dark' is presumably lower energy than light then yes it will have a different speed. |
Dark is about intensity, not wavelength - the speed of fewer photons will be the same as the speed of many photons. However if you are talking about standing waves, or interference effects, these can make the appearance of nodes or dark patches move at different speeds.