Infrared is a kind of electromagnetic waves in the waveband between visible light and microwaves. It is called visible light if it is in the range of 0.38~0.77μm, which is visible to the naked eyes. Infrared is in the range of 0.77~1,000μm, longer than the wavelength of visible light, and it’s classified into ‘near infrared’ in 0.77~3μm, ‘mid infrared’ in 3~6μm, ‘far infrared’ in 6~15μm, and ‘extreme infrared’ in the range of 15μm~1mm.
Spectrum distribution of visible light and infrared is as below picture.
All objects have their own temperatures over the absolute zero(-273.15℃, 0K), and emit electromagnetic waves corresponding to the temperatures. The higher the temperature is, the shorter the wavelength becomes and the more the radiant energy emits.For example, the objects whose temperatures reach to thousands of degrees like sun or filament of bulb emits visible light, while the ones near the normal temperature emit infrared. Even the objects thought to be cold like ice emit infrared. The hotter an object becomes, the more infrared it emits. The below image of the space shuttle which is landing shows that the bottom side is heated during the re-approaching to the atmosphere. The intensity of infrared measured corresponds to the temperature of the surface of an object as you can see in the infrared image of below human face.
IR image of a landing space shuttle
IR image of a human face
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