Thermal images of an object taken from a mirror can show temperature of that object. Which means thermal waves are reflected by the mirror. If so can heat energy from a hot object (not sunlight) be reflected by a mirror?
Ankush,
Heat can be radiated by infra-red radiation. Mirrors, such as polished metals, can reflect IR rays.
I am not entirely sure what you are asking.
Thank you for the answer.
Thant means mirrors can be used to transfer the heat.
Does that mean, mirror can be used to transfer the heat from room 2 to room 1? (Please refer to the illustration below)
Ankush,
Thermal radiation (i.e. longwave photons) will be reflected from room 2 to room 1 if the "mirrors" are propertly chosen. The efficiency depends on the reflectance of the mirrors in the LW spectral range (depending on the material, the reflectance may be high in the LW range and low in the visible range, or the opposite, or whatever else arrangement). It also depends on the "directional harvesting" of the radiation (diffuse reflectors will be less efficient than specular reflectors since a large part of the scattered light will be lost).
Hollow fibers internally coated with a high reflectance film are a typical example of a device used to convey thermal radiation from one point to another point and possibly in another direction.
B.R.
Heat is propagated by diffusive radiation and by EM emission propagated, what depends on propagating media. Nothing is defined for about the media in the question. Nevertheless the Planck law obeys.
Ankush,
The situation in your diagram is perfectly feasible.
And Mr Krapez hits the nail squarely on the head.
The fibre used to guide infra-red radiation from a CNC cutting laser is using a very large number of 'mirrors' (well, gradients in refractive index) to direct the heat radiation. So clearly infra-red rays can be reflected.
Many room heaters make use of the reflecting power of polished metal surfaces.
Thank you Krapez.
Use of concave mirrors will make any difference in transmission of heat from room 2 to room 1? And does that mean rooms with more mirrors will be warmer than rooms without mirrors?
Good to learn that Hollow fibers with a high reflectance film coating are used for heat transmission. Will be interesting to know where they exactly used?
Dear mr. Kawalli!
For far ifrared waves you need silver or (best) gold mirror with high reflection.
Silver
wavelength, mkm
0,247 0,3 0,35 0,4 0,5 1 2 3 4 5 6 7 8 9 10 11 12
R,%
26,6 17,6 82,7 93,8 96,2 97,9 98,6 98,7 98,7 98,8 98,8 98,8 98,9 98,9 98,9 98,9 98,9
Gold
wavelength, mkm
0,246 0,3 0,35 0,41 0,49 0,6 0,7 0,81 1 2 3 4 5 6 8 10 12
R,%
34,2 38,6 38,4 41 44,6 93,1 97,1 97,6 98,1 98,9 98,9 99 99 98,9 99 99 99,1
Ankush,
A classical example where a pair of parabolic mirrors is implemented is in photothermal radiometry where temperature over a small area has to be measured by an infrared detector. This area is put at the focus of the first parabolic mirror; the IR rays emitted by the small surface are thus redirected in a parallel beam; this parallel beam is then transformed into a converging beam by the second parabolic mirror and an IR sensor is put at the focus of the latter mirror. In this way, most of the radiation emitted by the surface of interest is received by the sensor (high efficiency, no problem of chromaticity as with refractive lenses).
Adding a mirror only modifies the angular distribution of the radiation. Soon or later the radiation will be absorbed by some surface (the walls and the mirror themselves since mirrors are not perfect). In the end the mirrors merely modify the spatial distribution of the radiation enery. The temperature distribution will be different but of course the total energy is conserved !
if you mean raddiation emitted due to the surface temperature, it does when it's different from the environment. When it's equilibrium with the environment it can not detected by a microbolometer-based radiometer .
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