Initially, the system of atoms is at thermal equilibrium, and the majority of the atoms will be in the ground state, i.e., N 1 ≈ N, N 2 ≈ N 3 ≈ 0. If the atoms are subjected to light of a frequency ν ν 13 = 1 h ( E 3 − − E 1 ) , the process of optical absorption will excite electrons from the ground state to level 3. This process is called pumping, and does not necessarily always directly involve light absorption; other methods of exciting the laser medium, such as electrical discharge or chemical reactions, may be used. The level 3 is sometimes referred to as the pump level or pump band, and the energy transition E 1 → E 3 as the pump transition, which is shown as the arrow marked P in the diagram on the right.