In 1913, Bohr gave another formula with a further different physical meaning to the quantity hν. In contrast to Planck's and Einstein's formulas, Bohr's formula referred explicitly and categorically to energy levels of atoms. Bohr's formula was W τ 2 − W τ 1 = hν where W τ 2 and W τ 1 denote the energy levels of quantum states of an atom, with quantum numbers τ 2 and τ 1. The symbol ν denotes the frequency of a quantum of radiation that can be emitted or absorbed as the atom passes between those two quantum states. In contrast to Planck's model, the frequency ν ν has no immediate relation to frequencies that might describe those quantum states themselves.