Light comes in discrete units called photons, and each photon has a particular energy associated with it. When a photon’s energy matches the energy spacing between two energy levels in an atom or molecule, it can cause a transition between these energy levels. This results in absorption of the photon, which transfers its energy to the atom or molecule. For example, the energy spacing between the ground and first excited electronic states of a hydrogen atom is 1.64 × 10⁻¹⁸ J which corresponds to a photon frequency of 2.47 × 10¹⁵ Hz. So photons with this frequency can excite the electron in a hydrogen atom from the ground to first excited electronic state.