In most theories before18th century, light was described as a substance composed of countless tiny particles. Because the particle theory cannot easily explain the refraction, diffraction and birefringence of light, Descartes (1637), Hooke (1665), Huygens (1678) and others put forward the wave theory of light. But at that time, due to the influence of Newton's authority, the particle theory of light still dominated. /kloc-At the beginning of the 9th century, Thomas Young and Fresnel's experiments clearly confirmed the interference and diffraction characteristics of light. By about 1850, the theory of light fluctuation has been completely accepted by academic circles. 1865, Maxwell's theory predicted that light was electromagnetic wave, and the experiment to confirm the existence of electromagnetic wave was completed by Hertz in 1888, which seemed to mark the complete end of the particle theory of light.

But the electromagnetic theory of light under Maxwell's theory can't explain all the properties of light. For example, in classical electromagnetic theory, the energy of light wave is only related to the energy density (light intensity) of wave field, and has nothing to do with the frequency of light wave; But many related experiments, such as photoelectric effect experiments, show that the energy of light has nothing to do with intensity, but only with frequency. A similar example is that in some photochemical reactions, the reaction will only occur when the light frequency exceeds a certain threshold, but it will not occur when the light intensity increases below the threshold.

At the same time, many physicists' research on blackbody radiation for more than forty years (1860- 1900) was terminated by the hypothesis established by Planck, who proposed that the emission or absorption frequency of any system is

The work of Einstein and others proved the existence of photons. The next question is: How to unify Maxwell's electromagnetic theory of light with the quantum theory of light? Einstein has never been able to find a theory to unify the two, but now the answer to this question has been included in quantum electrodynamics and its subsequent theory: the standard model.