Task: Text Categorization
Input (English): This value appears in both terms of the definition of the Lagrange invariant. Whether this value is in the numerator or the denominator of the expression for momentum is the focus of the Abraham-Minkowski controversy. The square of this value minus one all over the square of this value plus two is equal to four pi over three times number of molecules per unit volume times polarizability. The square of this value is equal to one plus a series of terms with B times lambda squared over lambda squared minus C according to the (*) Sellmeier equation. The difference between its ordinary and extraordinary versions gives birefringence. This value is on both sides of a law which relates the angle of incidence to another angle; that law is named for Snell. For 10  points, name this value symbolized n that is used to measure how much light bends when it moves between media.
Output (English): Science