Einstein (1905) successfully resolved this paradox by proposing that the incident light consisted of individual quanta, called photons, that interacted with the electrons in the metal like discrete particles, rather than as continuous waves. For a given frequency, or 'color,' of the incident radiation, each photon carried the energy E = hf, where h is Planck's constant and f is the frequency. Increasing the intensity of the light corresponded, in Einstein's model, to increasing the number of incident photons per unit time (flux), while the energy of each photon remained the same (as long as the frequency of the radiation was held constant). Einstein called this the photoelectric effect.
Quantum theory grew from einstien's photo electric effect. The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light. White light (what we call visible or optical light) can be split up into its constituent colors easily and with a familiar result - the rainbow. All we have to do is use a slit to focus a narrow beam of the light at a prism. This set-up is actually a basic spectrometer.