When an appropriate measure of energy say in the form of light of a certain wavelength, is supplied, electrons have a tendency to absorb the energy and get excited to a higher energy level. Conversely, if an electron is already at a higher energy state it will emit energy in the form of light (or heat) and return to a more stable lower energy state.
Thus, by recording the wavelength of light that is being absorbed (during excitation) or the wavelength being emitted (during de-excitation/relaxation), scientists can provide evidence of the movement of electrons from one energy level to the other.
