Given the task definition and input, reply with output. In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

The Sellmeier equation is an empirical relation describing the dependence of this quantity on wavelength. In left-handed materials, which have negative values of this quantity, the group and phase velocities point in opposite directions. Given by the square root of the product of the relative permittivity and permeability, for most materials it is approximately equal to the square root of the dielectric constant. FTP, name this quantity, equal to the speed of light over the local phase velocity, which appears in Snell's Law.
Science