Praxis Study Companion – Physics (5266) Practice Test

Light slows down when it moves from vacuum into a material, and the index of refraction quantifies that slowdown as a pure number. It’s defined as the ratio of the speed of light in vacuum to the speed of light in the material: n = c/v. This is dimensionless, and for ordinary materials v < c, so n is typically greater than 1. That slowdown is what causes refraction: when light hits a boundary, its frequency stays the same while its speed changes, which changes the wavelength and bends the path according to Snell’s law. The other forms don’t match this fundamental relation: they either mix in frequency or give a quantity with the wrong units, or they take the reciprocal of the correct ratio.