Atoms can absorb discrete amounts of heat or light at certain discrete wavelengths, corresponding to the energy requirements of the particular atom. When energy is added to the atom as a result of absorption of light, heat or collision with another particle (electron, atom, ion or molecule), one or more changes may occur. The energy absorbed may simply increase the kinetic energy of the atom or alternatively, the atom may absorb the energy and become excited. The permitted energy levels are finite and well defined, but an electron may be made to change to another level if the atom absorbs energy equal to the difference between the two levels. When this occurs, the electron moves to a higher energy level, such as E1. This atom is now said to be excited.