In the early 20th century, it was discovered that the interaction of light with Blank 1 is only possible at certain values depending on the substance. While for a gas the energy level diagrams appear as Blank 2 steps, for solids and liquids they are much more Blank 3 as they are subject to changes in pressure and temperature. The branch of physics called Blank 4 describes how the energies of level diagrams can be calculated.