折射角
the angle that a beam of light or other energy that is refracted by a surface makes with a line vertical to that surface
White light splits into a colour spectrum during refraction because each constituent wavelength's angle of refraction is different.
The letter 'r' here stands for the angle of refraction.
The angle of refraction can only be calculated for radiation that passes through a medium rather than radiation that is reflected off a surface.
Is the angle of refraction greater or less than the angle of incidence?
See also: refraction