Energy levels in hydrogen

If an electron from a low level is given energy it will be raised to a higher, or excited, level. This can be done electrically, by heat, by collision with another atom, by radiation or by a free electron hitting the atom. However the electron in the atom will only be excited if the energy of the incoming quantum of energy in whatever form is exactly the same as the difference in energy between the two levels.

When an electron falls from one level to another energy is emitted in the form of a quantum of radiation. The energy of this quantum and therefore its frequency and wavelength is determined by the difference in energy between the two levels.