and a state of (final) energy 
for: a quantum system making a transition between a state of (initial) energy and a state of (final) energy
relates: the frequency 
of the electromagnetic radiation emitted or absorbed by the system to the magnitude of the energy change 
[P8.2, P10.1, P10.2, P11.3]
may be written: 
where 
is Planck's constant. [P8.2, P10.2, P11.3]
results from: the combination of Planck's equation and the conservation of energy.
Copyright 1997, The Open University