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
One-way speed of light that slow clock-transport, the laws of motion, and the way inertial reference frames are defined, already involve the assumption of isotropic one-way speeds and thus are conventional as well. In general, it was shown that these experiments are consistent with anisotropic one-way light speed as long as the two-way light speed is isotropic. The 'speed of light' in this article refers to the speed of all electromagnetic radiation in vacuum. The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes