Speed of light difference in the speeds of photons of different energies, confirming that Lorentz invariance is verified at least down to the scale of the Planck length ("l" = ≈ ) divided by 1.2. In a medium, light usually does not propagate at a speed equal to "c"; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity "v". An actual physical signal with a finite extent (a pulse of light) travels
Speed of light an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1894, Paul Drude redefined with its modern meaning. Einstein used "V" in his original German-language papers on special relativity in 1905, but in 1907 he switched to , which by then had become the standard symbol for the speed of light. Sometimes is used for the speed of waves in "any" material medium, and for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature, has the same form as other related constants: namely, "μ" for the vacuum