How can reading the intensity of infrared radiation coming from an object let you calculate its temperature? Could any wavelength be used to measure temperature – why infrared, and not, for instance, green, or low frequency radio waves? And would an infrared thermometer give an accurate temperature of, say, an infrared LED?
[Physics] How / why do infrared thermometers work
infrared-radiationthermal-radiation
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The reason for this can be seen by examining how an infrared thermometer works. As you mentioned, it measures the infrared radiation, and uses this to determine the temperature. So, with that in mind, we consider what happens in the situations you mention. Namely, you cannot use it to measure the air temperature, because the emissivity of the air is piss-poor. So it will "think" the air is very cold, when it is not. Pointing at the sky, well, there's no IR emission of any great significance, that the device would register.
This can be seen by examining footage taken with a thermal infrared camera. This shows how the world "looks" in the thermal infrared band that these devices use. Using such a camera, one sees that the sky appears black. There is little emission, and there is practically no Rayleigh scattering of sunlight either at these wavelengths (it scatters better in the visible spectrum, esp. toward the blue, which is why the sky is blue).
For pointing at a far-off building, the distance means the amount of IR light reaching the device will be too small for it to make a proper reading.
Wrong:
"since infrared waves have a shorter wavelength"
Infrared has longer wavelength than visible and visible longer wavelength than ultraviolet .
White is a term for visible light mixed wavelengths. In the plot you can see that almost half of the sun's radiated energy arrives as visible light. The white buildings reflect this visible light which otherwise,impinging on the surfaces would be absorbed and turned into infrared by the interactions, adding to the arriving infrared.
What is absorbed and what is reflected depends on the chemical bonds of the surfaces, whether the incoming radiation can excite molecular states of the materials. Infrared is in frequencies/wavelengths of the black body radiation of bodies in the temperature ranges comfortable for the human body, so they easily raise the vibrational and rotational levels of solids and liquids and the kinetic energy of gasses.
37C curve seen here practically all in infrared, and lower temperatures more so.
Thus white paint will not reflect infrared as efficiently as visible, a large part of infrared will be absorbed as also some part of visible will scatter at the surface and degrade to infrared. Infrared can be reflected by metal mirrors, from the collective fields in metals . If you put aluminum foil in front of a heater you are sheltered from most of the heat which is reflected, but some of it is absorbed as can be seen by touching the foil.
If I expose an object to EM radiation only from the infrared spectrum, will it only reflect back infrared?
Yes, but most of it will be absorbed ( except by mirror metal surfaces) because the materials have the receptors for these wavelengths. This is due to the fact that larger wavelengths have photons with less energy which cannot excite higher energy levels.The energy of the photons goes as h*c/lamda where h is plancks constant, lamda the wavelength and c the velocity of light.
Is this true for other types of EM radiation?
No.Visible and ultraviolet by scatterings degrade their energy down to infrared frequencies, depending on the material.
Is it possible to make an object that looks white and absorbs a lot of infrared radiation?
Usually most of the infrared will be absorbed except by mirror metal surfaces.
If an object reflects most of the EM radiation that it receives of a particular wavelength λ, will it also reflect most of the radiation it receives of wavelengths less than λ (and absorb most of the radiation of wavelengths larger than λ)? Is this why objects that reflect most visible light (and hence look white) also reflect most infrared radiation (since infrared waves have a shorter wavelength)?
There is no such rule. It depends on the material and its chemical bonds.
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Best Answer
Conclusion: you can measure the temperature using the Planck's law by measuring emitted light at some wavelength (or many wavelengths for an accurate reading), only if the object is black at that wavelength. It's handy to do this in that portion of the infrared spectrum, where most materials have no special behaviour. In green wavelengths, it would only work for black, red and blue objects (but not for green/yellow/cyan/anything that reflects green). In radio wavelengths, the signals would be way too weak to detect with such a small sensor, and also, due to large wavelength, there's no way to target the measurement to a small object (you'd basically get an average of half the sky due to diffraction), and if an object itself is smaller than the wavelength, it's emission footprint is small (it's basically transparent for that wavelength, so the assumptions required for the Planck's law to work don't hold.