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I have never thought about it before as I got house with chimney just recently. And I have noticed, that simple metallic fireplace screen (which made as metallic grill/lattice) screens a really significant part of warm. At the same time it doesn't become hot.

I'm wondering, does it because it works as Faraday shield and screens in this way Thermal radiation, which is essentially is electromagnetic radiation, or I'm totally wrong?

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UPD: besides simple measurement of temperature in the room, I've made some experiment: I've measured heating of black circle on paper, hanged on same distance from fireplace with screen and without (of course there were two similar papers, not same one). I've measured heating using infrared thermometer. It shown me that with screen heating was only 1 Celsius degree on 2 minutes, while without screen - almost 3.5. Is it because of absorption/reflection?

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The wavelength of infrared is far too small to make the screen work as a Faraday shield. Instead, the individual wires cast shadows. The overall coverage does not seem all too large, however, so I don't expect much more than 10% to be absorbed/reflected. The grid, however, will provide a convection barrier for hot air.

So you have a layer of hot air and a layer of colder air starting with the grid. The transition to a more dense medium causes a certain amount of reflection to occur at the border.

For a similar reason one put radiators below single-pane windows: the layering of the moving sheet of warm air before the comparatively static cold air at the glass reflects warmth back into the room.

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  • $\begingroup$ That sounds reasonable. But just wondering: I've done some experiment, measured heating of black circle on paper, hanged on same distance from fireplace with screen and without (of course there were two same papers, not same one). I've measured heating using infrared thermometer. It shown me that with screen heating was only 1 Celsius degree on 2 minutes, while without screen - almost 3.5. Is it because of absorbtion/reflection? $\endgroup$ – Andremoniy Feb 13 '17 at 21:50
  • $\begingroup$ @Andremoniy Chances are that the screen is significantly altering air circulation around the fire: either the fire is burning more slowly with the screen there, or (worse) it's burning as fast but more of the heat is going up the chimney. $\endgroup$ – tfb Feb 13 '17 at 21:59
  • $\begingroup$ Well, I wouldn't say so. Lattice it self is quite thin, and I didn't notice any difference (visible of course) in burning with or without the screen. $\endgroup$ – Andremoniy Feb 13 '17 at 22:00
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Photons from the fire are not coherent, and their e-field does not re-inforce - the e-field transitions are around the waveleght of the blackbody radiation from the fire - around 50$\mu$m depending on temperature. There will be electron displacement, but the magnitude is much smaller and less coherent than normally described in relation to the Faraday Shield. Microwaves and radio waves have much longer wavelength at least 1mm.

Three means of transmission of heat; conduction, convection and radiation. Your fireplace uses all three to some extent. Colour has an impact on radiation absorption, black absorbing most.

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  • $\begingroup$ Ok, but why does not screen it self become hot in this case? Where does energy come if it prevents from heating room (comparing to state w/o screen)? Does it mean that it reflects it back? $\endgroup$ – Andremoniy Feb 13 '17 at 21:40
  • $\begingroup$ the screen absorbs the light, but it is not acting as a Faraday Shield when it does so. $\endgroup$ – JMLCarter Feb 13 '17 at 21:53
  • $\begingroup$ Yes, I understand what do you mean: it absorbs light, because it is black body. But in this case it should be quite hot and dissipate energy around. But room definitely becomes more warm without screen. So it doesn't. Where energy comes in this case? $\endgroup$ – Andremoniy Feb 13 '17 at 21:55
  • $\begingroup$ As well as gaining heat from the fire it is also loosing heat. As it is cool it is loosing more heat than it is gaining - which is what you want in a fireguard. It is loosing heat by each of the three heat transport methods; but most significant will probably be convection. A $\endgroup$ – JMLCarter Feb 13 '17 at 22:07
  • $\begingroup$ It should heat room almost same in this case as it would be removed, right? $\endgroup$ – Andremoniy Feb 13 '17 at 22:08

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