Newton did a prism beam splitting experiment a long time ago. Visible light (at that time, Newton's experimental object was natural visible light, that is, the visible part of sunlight, and actually there were invisible parts of other wavelengths, which were just invisible to the naked eye, so he didn't realize it) turned into color through the prism, forming the most primitive spectrogram. The so-called spectrogram, in short, is to mark the light with different frequencies on the graph with the wavelength as the horizontal axis, just like a menu list, except that different dishes are replaced by frequencies arranged according to size. Newton, based on the observation of the geometric properties of light at that time, proposed that light is a particle, so small that the naked eye can't distinguish a single light particle, and light is composed of light particles with different sizes and fast vibration. In the study of light in early classical physics (compared with quantum physics developed later), interference and diffraction phenomena are the most powerful evidence of the existence of light as a wave, which was recognized by most physicists at that time, among which Fresnel diffraction and Young's double-slit interference are the most important (mentioned in general optical books). From Newton's particles to Huygens' wave theory, the debate about the nature of light lasted for more than a century. Classical physics cannot accept that light is both a wave and a particle.
In the later stage of the development of classical electromagnetism, Maxwell's equations (see Maxwell's equations) unified the electromagnetic field theory and explained the geometric characteristics of light mathematically. The fluctuation of light establishes a complete system from theory to experiment. Maxwell pointed out that electromagnetic waves and light have the same speed, and predicted that light (which was limited to the concept of visible light at that time) was electromagnetic waves. Classical physics seems to have come to an end. 1At the end of the 9th century and the beginning of the 20th century, many scientists thought that the exploration of the physical world had been basically completed, and all that remained was to supplement and refine the work.
But history has brought relativity and quantum mechanics. As early as 1888 Hz, it was observed that when ultraviolet rays hit the metal, it would make the metal emit charged particles. Thomson and others confirmed that such charged particles are electrons, which is the so-called photoelectric effect. After Planck put forward the concept of quantization, Einstein successfully explained the photoelectric effect. With the deepening of the understanding of chemical elements and the exploration of atomic models, Bohr, the representative of the Gebenhagen School in Denmark, took a pioneering step towards quantized atomic physics and spectroscopy-the quantum model of electron orbits (the word "quantum" can be simply understood as "discontinuity", as opposed to various continuous processes in the macro world). Physicists realize that electrons in atoms jump from one orbit to another, accompanied by changes in electromagnetic energy, which appear in the form of electromagnetic wave radiation or absorption with different wavelengths and frequencies.
From 1923 to 1924, the young French scientist de Broglie put forward the hypothesis that all matter particles have fluctuation and particle duality in his doctoral thesis entitled "Research on Quantum Theory". It is pointed out that the wavelength (fluctuation category) of matter particles is inversely proportional to the particle momentum (particle category). This is the first time to link the particle nature and fluctuation of light, and it has been proved to be successful. Later, it was found that the electron group also fluctuated, so the particle nature of electromagnetic waves was confirmed.
Simply put, light can be represented as photons or electromagnetic radiation. For the explanation of the English version, please refer to Wikipedia and Encyclopedia Britannica online. In-depth theory can refer to Atomic Physics (written by Yang Jiafu) and some modern optical books, and a deeper understanding should refer to quantum mechanics, electrodynamics and quantum electrodynamics.