You are aware that light could be called a ‘wave’ or ‘particle’. According to particle theory, light consists of many packets of energy called photons. These photons may be generated by chemical reactions or heat.
Einstein discovered that electrons were emitted from a metal surface when it was hit by ultraviolet light. This is called photoelectric effect. UV light is found in sunlight and has a wavelength shorter than that of visible light. Assuming light contains photons they transfer their energy to electrons, which knocks off from their "parent" atoms. Once freed, the electrons move along the metal or get ejected from the surface.
When an electron from higher-than-normal orbit falls back to its normal orbit, it gives off energy in the form of a photon. This photon has a very specific characteristic - the frequency (color) exactly matches the distance the electron falls.
This phenomenon is used in lamps that are used for street-lighting purposes.
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