A section of the wavelength from 0.76 to 400 microns in the spectrum is called infrared light, and infrared rays are invisible light. All substances above absolute zero (-273 ° C) can produce infrared light. Modern physics is called heat ray.
Infrared can be divided into two categories: near infrared and far infrared. Near-infrared or short-wave infrared, with a wavelength of 0.76-1.5 microns, penetrates deep into the human body, about 5-10 mm; far-infrared or long-wave infrared, with a wavelength of 1.5-400 micron, is absorbed by the superficial skin, and the penetration depth is less than 2 mm.
Have a thermal effect
2. Strong ability to penetrate clouds (long wavelength, easy to diffract)