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 we now subject the atoms to light of a frequency formula_7, 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.