Spectral properties of materials
Spectro-imagery Principle and applications

When an atom or molecule interacts with an incident electromagnetic wave of wavelength λ = 1/ν where ν is the wavenumber, quantum mechanics tells us that the atom or molecule absorbs the incident radiation by passing from an energy level E ₁ , associated with state 1, to an energy level E ₂ , associated with state 2, only if the tuning condition is satisfied :

Energy levels of the order of a few eV correspond to wavelengths...