Why does production of white light using a LED require combining a short wavelength LED such as blue or UV, and a yellow phosphor coating?
Why can't a single LED produce pure white light?
[Physics] Why can’t single LEDs produce white light directly
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Best Answer
White light does not have a single frequency, it is a mixture of all of the colors. Similarly pink does not have a single frequency it is a mixture of red with white i.e. white with extra red.
If the color does not appear in the color spectrum (rainbow) then it is not a single frequency but a mixture of different frequencies.
In an LED light is produced by an electron dropping from an excited state to a relaxed energy state which produces a light of a given energy (frequency) however all of the light is going to be of the same color since the bandgap (energy difference) is going to be the same for all of the electrons.
It is possible (see here) to put three different LEDs on one chip corresponding to Red Green and Blue. This produces the effect of a white light. But is not truly white since it only contains Red Green and Blue and no Orange Yellow or Violet / purple.
Your computer screen cannot produce true whites either for the same reason. If you see yellow on your computer screen, there is not light of the frequency yellow only the right amount of Red Green and Blue to convince your eyes nerves to fire the same as if it were a true yellow frequency.