In physics, the process of emission and transmission of energy in the form of electromagnetic waves or photons is called electromagnetic radiation. Light is a part of electromagnetic radiation, which is electromagnetic radiation with wavelengths between the transition region to X-rays (λ≈1 nm) and the transition region to the radio band (λ≈1 mm). Not all of these lights can be seen, and only a part of them can be seen by the human eye. This part of the light is called visible light, and its wavelength range is usually limited to between 380 and 780 nm. Light waves occupy only a small fraction of the entire electromagnetic spectrum, as shown in Figure 1. In visible light, the shortest wavelength is violet light, the slightly longer wavelength is blue light, and the following order is cyan light, green light, yellow light, orange light and red light. Among them, red light has the longest wavelength.

Optical radiation with wavelengths longer than visible radiation is called infrared radiation. The wavelength range is

Infrared radiation between 780nm~1mm is usually subdivided into: IR-A780~1400nm; IR-B1.4~3μm; IR-Cμum~1mm.

Optical radiation with wavelengths shorter than visible radiation is called ultraviolet radiation. Ultraviolet radiation in the wavelength range of 100~400nm is usually subdivided into: UV-A315~400nm; UV-B280~315nm; UV-C100~280nm.

From a physical point of view, these theories basically say that light is a type of energy that is delivered. At present, scientists use two theories to explain the nature of light, which are “electromagnetic wave theory” and “quantum theory”. Electromagnetic wave theory holds that luminous bodies emit light in the form of radiant energy, which in turn transmits outward in the form of electromagnetic waves. Electromagnetic waves act on the human eye to produce the sensation of light. Quantum theory holds that luminous bodies emit radiant energy in discrete “beams” that are emitted in straight lines and act on the human eye to produce the perception of light. The motion of light in space can be satisfactorily explained by the electromagnetic wave theory. The effects of light on objects (eg, on barrier photocell photometers) can be satisfactorily explained by quantum theory.

All forms of radiant energy travel in a vacuum with the same speed of 299,793 km/s (closer to 3 × 105 km/s). When radiant energy passes through a medium, its wavelength and speed change; while the frequency is determined by the source of the radiation that produces the electromagnetic wave, it does not change with the medium it encounters. The velocity of the radiant energy can be determined, and the relationship between frequency and wavelength can be expressed by the following equation:

where v——the propagation velocity of the wave in the medium, m/s;

n is the refractive index of the medium;

λ——wavelength in vacuum, m;

v – the frequency of the photon, Hz.

Table 1 gives the speed of light in different media, and its frequency corresponds to the frequency of light with a wavelength of 589 nm in air.