“But in Bohr’s model, where only certain electron orbits were allowed, there was an immediate explanation. Even though electrons couldn’t linger in between the allowed orbits, they could jump from one to another. An electron could fall from a higher-energy orbit to a lower-energy one by emitting light with just the right energy to compensate, or it could leap upward in energy by absorbing an appropriate amount of energy from ambient light. Because the orbits themselves were quantized, we should only see specific energies of light interacting with the electrons. Together with Planck’s idea that the frequency of light is related to its energy, this explained why physicists saw only certain frequencies being emitted or absorbed.”