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 rotation, the speed of light can be calculated. The method of Foucault replaces the cogwheel by a rotating mirror. Because the mirror keeps rotating while the light travels to the distant mirror and back, the light is reflected from the rotating mirror at a different angle on its way out than it is on its way back. From this difference in angle, the known speed of rotation and the distance to the distant mirror the speed of light may be calculated. Nowadays, using oscilloscopes with time resolutions of less than one nanosecond, the speed of light can be directly measured