The light that the human eye can see is called visible light, which is mainly concentrated in the spectral band from 0.38 micron to 0.78 micron. It can be subdivided into seven colors: purple, blue, cyan, green, yellow, orange and red. So there are no other lights after the red light? Actually, it is not. After red light, the frequency is infrared for a long time, but it is invisible to human eyes. 1800, when British physicist Herschel was studying the heat of various colors of light, he deliberately blocked the only window in the darkroom with a wooden board, and opened a rectangular hole in the board with a beam splitting prism in it. When sunlight passes through this prism, it is broken down into colored bands. In the experiment, he suddenly found a strange phenomenon: the thermometer placed outside the red band has a higher indicating value than other thermometers in the room. After many experiments, this so-called hottest high temperature zone is always located outside the red light at the edge of the light band. So Huxley announced that in addition to visible light, there is an invisible "hotline" in the light emitted by the sun. This invisible "hotline" is located outside the red light, so it is called infrared.

Infrared ray is actually an electromagnetic wave, and its wavelength is between 0.78 micron and 1000 micron. For the convenience of research, scientists divide infrared into three bands: near infrared: the wavelength is 0.78 ~ 3.0 micron, middle infrared: the wavelength is 3.0 ~ 20 micron, and far infrared: the wavelength is 20 ~ 1000 micron.

The discovery of infrared marks another leap in human understanding of nature.

Infrared rays are also electromagnetic waves.

As we all know, radio can emit sound when receiving electromagnetic waves, and TV can display images when receiving electromagnetic waves. Colorful light in nature is electromagnetic wave. Experiments show that infrared rays are also electromagnetic waves.

Then, what is the difference between electromagnetic waves received by radio and television, light received by human eyes and infrared rays? The fundamental difference lies in different wavelength ranges.

The alternating current we often use also propagates along the conductor in the form of electromagnetic waves. Its wavelength is 6000 kilometers, which can be said to be the longest electromagnetic wave. The wavelength of electromagnetic waves received by the radio varies from several hundred meters to several tens of meters, which is called medium wave or short wave. The wavelength of electromagnetic waves received by TV sets varies from several meters to several centimeters, which is called microwave. The shortest wavelength of radio waves is only a few millimeters. Infrared is also an electromagnetic wave with a wavelength ranging from 0.78 micron to 1000 micron. 1 micron is equal to one thousandth of a millimeter. In order to facilitate research, infrared can also be divided into the following three bands:

Near infrared: the wavelength is 0.78 ~ 3.0 micron.

Mid-infrared: the wavelength is 3.0 ~ 20 microns.

Far infrared: wavelength 20 ~ 1000 micron.

Electromagnetic waves with a wavelength less than 0.78 micron are visible light. The wavelength range of visible light is 0.38 micron to 0.78 micron. Visible light with different wavelengths has different colors, and the relationship between wavelength and color is shown in figure 1.

Electromagnetic waves with shorter wavelengths than visible light include ultraviolet rays, X rays, gamma rays and cosmic rays. Infrared rays and radio waves, visible light, ultraviolet rays and various rays form a continuous electromagnetic spectrum.

Characteristics of infrared ray

Theoretical analysis and experimental research show that not only does infrared exist in sunlight, but any object with absolute zero temperature (such as human body) is constantly radiating infrared. It is ice and snow, because their temperature is also very high, which is absolute zero, so they are constantly radiating infrared rays. So the biggest feature of infrared ray is that it is ubiquitous in nature. In other words, any "hot" object can radiate infrared rays although it does not emit light. So infrared ray is also called thermal radiation, referred to as thermal radiation.

Compared with visible light, another feature of infrared ray is its rich and colorful colors. Because the longest wavelength of visible light is 1 times (780 nm ~ 380 nm) of the shortest wavelength, it is also called octave. The longest wavelength of infrared ray is 10 times of the shortest wavelength, which means it has 10 octaves. Therefore, if visible light can display 7 colors, infrared light may display 70 colors, showing rich colors.

Infrared ray has a good performance of penetrating smoke, which is another feature of it.