What is LEDs?

A light-emitting diode (LED) emits a quasi-monochromatic band a few tens of nanometers wide: narrow enough to look a definite color, far broader than a laser line. A white LED is a blue LED (peak near $450\,\mathrm{nm}$) coated with a phosphor, a material absorbing part of the blue and re-emitting it as a broad yellow band: blue and yellow being complementary (Proposition 11.5), the mixture reads as white.