What is the form of light?

Strictly speaking, light is a kind of radiation that can be observed by human eyes. Experiments show that light is electromagnetic radiation, and the wavelength range of this part of electromagnetic wave is about 0.77 micron of red light to 0.39 micron of purple light. Electromagnetic waves with a wavelength of more than 0.77 micron to about 1000 micron are called "infrared rays". Below 0.39 micron to about 0.04 micron is called "ultraviolet". Infrared and ultraviolet rays can't cause vision, but the existence of this luminous object can be measured and detected by optical instruments or photography. Therefore, the concept of light in optics can also be extended to infrared and ultraviolet fields. Even X-rays are considered as light, and the spectrum of visible light is only a part of the electromagnetic spectrum.

Light has wave-particle duality, that is, it can be regarded as a kind of high-frequency electromagnetic wave (10/2 ~1015hz), and it can also be regarded as a particle, that is, photon for short.

The speed of light replaced the platinum watch kept in the International Bureau of Metrology in Paris as the standard for defining "meter", and it was agreed that the speed of light was strictly equal to 299,792,458 m/s, which was consistent with the definition of meter and second at that time. Later, with the continuous improvement of the experimental accuracy, the numerical value of the speed of light changed, and the meter was defined as the distance traveled by light in 1/299792458 seconds.