The lowest lying excited state of sodium is 2.104 eV above the ground state. Given that Planck's constant is 4.135x10 (to the negative 15 pwr.) eV/Hz, what color of light do you expect sodium atoms to give off? Table salt is the crystal sodium-chloride NaCl. If you have a gas burner in your kitchen, take some salt and throw it onto the flame. The flame will excite the sodium atoms to this excited state, and they will then fall back down to the ground state giving off a spontaneous emission photon. What color do you see form the salt in the flame? Does it agree with your prediction?
To figure out the color of some light, you need to know what its wavelength or frequency is.
<br>Fortunately, this is easy to figure out if you know how energetic the light is. You simply use the formula Max Planck devised to relate the energy ...
This job determines salt color in a problem.