I am currently reading about matter waves and electron traps. the textbook states, by the equation: hf = [tex]\Delta[/tex]E = Ehigh - Elow If an electron confined to a one-dimensional infinite potential well is to absorb a photon, the energy hf of the photon must equal the energy difference [tex]\Delta[/tex]E between the initial energy level of the electron and a higher level. Then as an aside, the textbook states: Although the above equation and what we have discussed about photon absorption and emission can be applied to physical (real) electron traps, they actually cannot be applied to one-dimensional (unreal) electron traps. The reason involves the need to conserve angular momentum in a photon absorption or emission process. I do not understand this statement about angular momentum.