What is the quantum phenomenon of light

While wave theory perfectly explains the propagation of light, it cannot accurately describe the emission and absorption of light. Experiments have shown that when light (or other forms of electromagnetic radiation) is emitted or absorbed, it involves a discrete amount of energy called a "quantum." A quantum of light energy is called a photon, and its energy, Q, is given by the Planck relation:

Q=hv

where v is the frequency and h is a constant, known as the Planck constant. A photon of visible light has very little energy. In the blue-green part of the spectrum, a photon with a wavelength of 500 nanometers has an energy of 4x10⁻⁹ joules. One watt of radiation at this wavelength is equivalent to 2.5x10⁻¹⁸ photons per second. Based on this quantum relation, Planck theoretically derived in 1901 the continuous spectral distribution of radiation from a perfect radiator, or "blackbody." A blackbody is an object that completely absorbs all the radiation it receives. The spectral distribution of incandescent solids (such as tungsten used in filaments) is similar to that of a blackbody, but with important differences. The discontinuous distribution obtained when a gas is excited by electricity can also be explained by quantum theory.