Task: Text Categorization
Input (English): The Sellmeier equation relates it to wavelength for each absorption resonance, taking into account the dispersive effects of the medium. It can be defined for the group velocity as well, but is usually defined for the phase velocity so as to keep track with the actual motion of the medium itself. Negative values have recently been found as a result of exotic metamaterials, and the value itself is commonly expressed as the geometric mean of the permittivity and permeability of the medium. For 10 points, name this constant which describes how the speed of light is affected by a medium.
Output (English): Science