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
Max Planck Institute for the Science of Light Planck Society. In June 2008, after an evaluation, the senate of the Max Planck Society decided to convert this group to a Max Planck institute and thereupon founded the "Max Planck Institute for the Science of Light" (German "Max-Planck-Institut für die Physik des Lichts"). with effect from January 1, 2009. In 2016, the new building was opened. The project was made by the Munich architecture bureau "Fritsch + Tschaidse Architekten GmbH". Max Planck Institute for the Science of Light The Max Planck Institute for the Science of Light (MPL) performs basic research in optical metrology, optical communication, new optical materials,