1. extends in a straight line from the center in all directions.
2. It refers to the propagation of electromagnetic waves such as thermal radiation, light and radio waves, also known as radiation.
When the display emits light, the outer electrons of the fluorescent screen are excited to emit X-rays.
definition
All objects in nature always transfer heat to the outside world in the form of electromagnetic waves as long as the temperature is above absolute zero. This way of transmitting energy is called radiation. The energy released by an object through radiation is called radiant energy. Radiation is calculated in roentgen per hour (r)
An important feature of radiation is "equivalence". Objects (gases) radiate outward regardless of temperature. Object A can radiate to object B, and object B can also radiate to object A ... This is different from conduction, which is unidirectional. People who are exposed to a lot of radiation should wash their whole bodies thoroughly with soap and plenty of water, and seek the help of doctors or experts immediately! (The picture shows the warning sign "Danger of radioactive materials, beware of radiation")
When radiant energy is absorbed by an object, it will produce a thermal effect. Different radiant energy absorbed by objects leads to different temperatures. Therefore, radiation is an important way to convert energy into heat. Radiation heat transfer is a process of transferring heat between hot and cold objects by electromagnetic radiation, which belongs to non-contact heat transfer and can also be carried out in vacuum. Theoretically, the electromagnetic waves emitted by objects are distributed in the whole spectral range, but in the temperature range encountered in industry, the practical significance is the thermal radiation with the wavelength of 0.38 ~ 1000 micron, and in the infrared (also known as thermal radiation) section, most of them are in the range of 0.76 ~ 20 micron. The so-called infrared heating is to use the thermal radiation of this section. It is very important to study the law of thermal radiation for the reasonable design of heat transfer in the furnace, and it is also of positive significance for the labor protection of operators in the high temperature furnace. When a system needs heat preservation, even if the temperature of the system is not high, the influence of radiation heat transfer can not be ignored. For example, silver plating on thermos bottles is to reduce the heat loss caused by radiation heat transfer. The basic concept of thermal radiation Any object continuously absorbs the radiant energy of the surrounding objects while emitting radiant energy. The difference between the energy radiated by an object and the energy absorbed is the net energy it transmits. The radiation ability of an object (that is, the energy radiated from the surface per unit time) increases rapidly with the increase of temperature. Generally speaking, when an object is radiated by other objects (energy is Q), the part that is absorbed and converted into heat energy is QA, the part that is reflected is QR and the part that passes through the object is QD. Obviously, these parts are related to the total energy: QA+QR+QD=Q If A=QA/Q is called absorption rate, R=QR/Q is called reflectivity, and D = QD.
If A= 1 and R=D=0 of an object, that is, the thermal radiation energy reaching the surface of the object is completely absorbed, then the object is called an absolute blackbody, or blackbody for short. If R= 1 and A=D=0, that is, all the thermal radiation energy reaching the surface of the object is reflected; When this reflection is regular, this object is called a mirror; If it is random reflection, it is called absolute white body. If D= 1 and A=R=0, that is, all the energy of thermal radiation reaching the surface of the object passes through the object, which is called a diatherm. In fact, there is no absolute blackbody or absolute white body, only some objects are close to absolute blackbody or absolute white body. For example, the surface of dark black paint is close to blackbody, and the absorption rate is 0.97 ~ 0.98; The polished copper surface is close to the white body, and the reflectivity can reach 0.97. The absorption and reflection characteristics of solid surface are mainly influenced by surface condition and color, and the influence of surface condition is often greater than color. Solids and liquids are generally unaffected by heat. The energy of thermal radiation is completely absorbed only after passing through the solid or liquid surface for a short distance (generally less than 1mm, and after passing through the metal surface 1μm). Gases have little ability to reflect thermal radiation energy. Monoatomic and symmetric diatomic gases (such as argon, helium, H2, N2, oxygen, etc. ) can be regarded as a diathermant at general temperature, while polyatomic gases (such as CO2, H2O, SO2, NH3, CH4, etc. ) has a considerable absorption capacity in a specific wavelength range.
Radiation is emitted in the form of electromagnetic waves and particles (such as alpha particles and beta particles). Radio waves and light waves are electromagnetic waves. Their propagation speed is very fast, which is the same as that of light wave in vacuum (3× 10 10 cm/s), but slightly slower in air.
Electromagnetic wave is a composite wave composed of waves with different wavelengths. Its wavelength ranges from cosmic rays of10e-10μ m (1μ m =10e-4cm) to radio waves of several kilometers wavelength. X-rays, X-rays, ultraviolet rays, visible light, infrared rays, ultrashort waves and long-wave radio waves all belong to the scope of electromagnetic waves. Visible to the naked eye is a short electromagnetic wave, which is called visible light from 0.4-0.76 micron. Visible light, after being split by a prism, becomes a light band composed of seven colors: red, orange, yellow, green, cyan, blue and purple. This band of light is called spectrum. Among them, the wavelength of red light is the longest, the wavelength of purple light is the shortest, and the wavelengths of other colors are in between. There are infrared and radio waves whose wavelength is longer than red light (>: 0.76 micron); The wavelength is shorter than violet light (
The wavelength of solar radiation is mainly 0. 15-4 micron, and the maximum radiation wavelength is 0.5 micron on average. The radiation wavelength of ground and atmosphere is mainly 3- 120 micron, and the maximum radiation wavelength is 10 micron on average. It is customary to call the former short-wave radiation and the latter long-wave radiation.