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
Light-year similarly be formed usefully by multiplying units of time by the speed of light. For example, the light-second, useful in astronomy, telecommunications and relativistic physics, is exactly metres or of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used in popular science publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time. Light travels approximately one foot in a nanosecond; the term "light-foot" is sometimes used as an informal measure of time. Light-year The light-year