$($eV$)$ When an electron is in a higher energy state it may spontaneously emit a photon and return to its lower energy state. These photons carry an energy equal to the energy difference between the levels. $19.$ If an electron is in the n $=4$ state which states can it $$fall down$$ to by emitting a photon? $20.$ What are the photon energies that correspond to the above transitions? $($eV$)$ For a photon E $=$ hc$/$ where h $=4\mathrm{.}1357$ x $10-15$ eVs is the Planck$$s constant, c $=3\mathrm{.}0$ x $108$ m$/$s is the speed of light and is the wavelength. $21.$ What is the wavelength of the photons found in question $19?($nm$)]$