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Aluminum foil is said to be not absorbing light at all. It reflects light. So, does it mean that a more shiny aluminum foil will reflect more light and thus make the room more cooler as compared to less shiny foil?

If yes, then where does unreflected light go in case of less shiny aluminum foil?

Does less shiny and more shiny actually make a difference? Why?

In order to keep whole room cool will it help if I stick the foil to the outside of wooden door (exposed to the sunlight) too?

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Please elaborate on where and how the foil is being placed. Is it only on the windows? Is the whole room coated in foil? You might be interested in searching for "optical cavity" on the site. – Brandon Enright May 8 '13 at 15:39
@BrandonEnright the foil is only on the outside of glass windows. – TheIndependentAquarius May 27 '13 at 4:13
If your room is hot because the air is hot, then adding aluminium reflectors won't help. It's only going to prevent direct sunlight to heat up the room. – fffred May 27 '13 at 7:50
@fffred The day time temperature here is 43 degree Celcius. – TheIndependentAquarius May 27 '13 at 7:53
@user462608 If you measured this temperature outside in the shade, then it means that's the temperature of air. You will not get below this temperature using aluminium foil. If your room is hotter than 43 C because of direct sunlight, then you might get it closer to 43 C, but I would suggest you simply open the windows ... – fffred May 27 '13 at 7:57

2 Answers 2

First, there's no perfect reflector nor absorber. In fact - even Aluminium does absorb some radiation (by which it gets heated, can be noticed at incident high frequency radiation). One more thing is that aluminium foils are designed in a way to reflect light.

Here's the Wiki article quote...

Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass. It is difficult to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny. This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe that the different properties keep heat out when wrapped with the shiny finish facing out, and keep heat in with the shiny finish facing inwards, the actual difference is imperceptible without instrumentation. The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%.

The shiny and not-shiny surfaces are totally a favor of production technology (credit goes to the rollers). Now, to the "why" question.

As a physics parameter, we use reflectivity to address the shininess. As we can see, the reflectivity is quite high for the bright surface, compared to dull one. The unreflected light (as you say) can go anywhere. It can go inside the aluminium foil (i.e) it's absorbed and hence the 12% & 20% loss...

Response to comment (based on edit): That's a nice strange idea. With some perfection (I mean, there should be very less allowance of any sorta radiation inside), the room will be relatively cooler. But, in reality (where we can't expect idealistic things), there will always be some radiation inside. But, it keeps the room warmer compared to the outside. But, always be careful when playing with such things because, any sort of harmful radiation (if any- what about a heater or even an electric iron?) inside the room will be reflected back to you by the matte side - which can be very harmful...

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Thanks for the answer. – TheIndependentAquarius May 27 '13 at 4:13
@user462608: You're welcome ;-) – Waffle's Crazy Peanut May 27 '13 at 4:14
One more question: In order to keep whole room cool will it help if I stick the foil to the outside of wooden door too? – TheIndependentAquarius May 27 '13 at 4:16
@user462608: I've revised my answer based on your edit... – Waffle's Crazy Peanut May 27 '13 at 4:30

It depends where the aluminum foil is situated.

For example I use aluminum foil behind a wood stove to reflect the heat to the room instead of heating the wall.

So, does it mean that a more shiny aluminium foil will reflect more light and thus make the room more cooler as compared to less shiny foil?

You can see from Crazy Buddy's answer that the more mirror like the surface the less is absorbed by the foil.

If yes, then where does unreflected light go in case of less shiny aluminium foil?

The unreflected light is absorbed and ends up in heating the foil itself.

Thus, if you have an aluminum foil on a sun side of a wall you have to leave air between the wall and the aluminum so as not to get that residual heat heat the wall by contact. Air is a bad conductor of heat.

You could have it on an inside wall if you want to reflect back all the energy and have few heat losses. Again an air gap will help keeping the heat in the room instead of heating the wall and radiating energy out.

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Thanks. Actually I do NOT want the room to get heated at all. Currently the foil is tuck to the outer surface of glass windows. – TheIndependentAquarius May 27 '13 at 4:15
Then it would be a good idea to have a layer of insulator to the glass, so the contact of foil to glass is insulated thermally. The the residual non reflected energy will be mostly radiating out. As I said air is a good insulator.If you stretch simple foil across the frame then there will be air in between glass and foil. – anna v May 27 '13 at 4:21
Then it would be a good idea to have a layer of insulator to the glass Didn't get this. what am I supposed to do? Please see the additional question this OP – TheIndependentAquarius May 27 '13 at 4:22
Think of a dewar. It often is metallic outside, then there is a layer of vacuum to stop contact heat transfer ( insulation) and then the container that needs to be kept cool. If you want a whole room to be kept cool you could mimic this. The reason is that the metallic surface gets heated by the non reflected energy and radiates +transfers by contact. Insulation stops that transfer . – anna v May 27 '13 at 4:27
@user462608 take a sheet of bubble wrap used in packaging, fix this on the window between the aluminium foil and the glass. The aluminium heats up from the 20% of the sun it doesn't reflect, the air in the bubble wrap stops this conducting to the glass – Martin Beckett May 27 '13 at 14:59

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