Electrons in atoms and molecules have fixed energy levels, according to the principles of quantum physics. When there are transitions among different energy levels, light with certain frequencies is emitted or absorbed. This allows atoms and molecules to be analyzed with the help of the absorbed light's spectrum. In the 1960s Nicolaas Bloembergen used laser light, which has waves in phase and of the same wavelength, to determine energy levels with great precision. By coordinating three laser waves, a fourth laser wave was created, and a larger part of the spectrum could be covered.