When light passes from one medium into another, its speed typically changes and consequently, its wavelength, as well, with its frequency remaining unchanged.

The index of refraction, n, of a medium is defined as the ratio of the speed of light in that medium, v, to the speed of light, c, in a vacuum; that is, n = c/v.

Recalling the relationship involving wavelength, frequency and propagation speed, now imagine that red light (frequency ~4.3x1014 Hz) with a wavelength of ~700 nm in a vacuum, enters a glass block. You determine that its wavelength in the glass shortens to ~467 nm. You can then conclude that the index of refraction of the glass for that red light is