# What determines the color of photon that is emitted from an exited atom?

I understand the principle of how light is emitted from an atom. What I don't know is why neon atom is red and copper is green when exited? Is is the distance between the electron to protons or the number of electrons? When halogen bulbs get old the gas ionizes and emits a purplish hue compared to its normal white. Would a neon atom emit a different color if ionized?

• The color of light emitted is dependent on the difference in energy leve!s of the electron before and after it "jumps" and emits the photon. So it varies from atom to atom. – user108787 Apr 29 '16 at 23:35

The color of the photon is related to its frequency $f$, which can be related to the energy of the photon by the expression $E = hf$, where $h$ is Planck's constant. Thus the different colors of the emitted photons describes their different energies.
• "The color of the photon is related to its frequency $f$", is reasonable true, but only because you restrict it to light of a single frequency by considering only one photon. Light including contributions of multiple frequencies does not follow so simply a understanding because "color" is a sensory result rather than a physical property of the light and the processing your brain does on visual input is considerable. – dmckee Apr 30 '16 at 0:08