What's the difference between i.e. and e.g.?
To limit a variable or variables describing a physical system to discrete, distinct values. For example, the energy of electromagnetic radiation such as light at a given frequency must be an integer multiple of hν, where ν is the frequency and h is a Planck's constant; electromagnetic energy is thus inherently quantized (in this case, photons are the quanta of energy). The distinct orbitals of electrons in an atom are also a case of quantized energy. Many apparently continuous phenomena turn out to be quantized at a very fine level or very small scale; quantum mechanics was developed in large part to explain many unexpected cases of quantization in the natural world.