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Aluminium foil is widely used for thermal isolation. As far as I know it reflects the thermal infrared radiation. Also I've seen a lot of guides about strengthening WiFi signal by putting the electromagnetic shield at not needed sides of WiFi antenna, so that EM waves are reflected to the useful zone.

I'd like to know how exactly it works though.

In Wikipedia it is written that usual household aluminium foil has thickness of about 16 microns, and heavy duty foil – 24 microns. The thermal infrared light has wavelength of 3-30 microns. Does it mean that, for example, 16 microns thick foil will reflect only light with wavelength up to 16 microns, and all light with larger wavelength will pass through such foil freely?

WiFi has frequency 2.4 GHz which corresponds to wavelength of 12.5 cm. Then such long waves shouldn't be affected my the foil that has thickness 10000 times smaller.

Or does it have to do with the fact that aluminium is a conductive material, and any electromagnetic waves, regardless of wavelength, can't pass through the conductive material. If this is the case, then how does the reflection work?

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It's all to do with the conductivity of the material, the thickness is only a secondary effect because as you reduce the thickness you reduce the conductivity.

The only time that thickness has a direct effect is if you make a surface from layers of a transparent material which are a precise fraction of a wavelength in order to use interference effects to increase or decrease reflectivity.

The details are little different for light and radio frequencies but you can calculate the 'depth' the waves penetrate into the metal quite easily - skin effect

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Great. This is clear now. But how does it reflect the wave? Am I right thinking that the layer of atoms of Aluminium absorbs the EM wave, and then reemits the same wave in all directions, which means that 50% of wave's energy will be reflected back? And due to the presence of multiple layers of aluminium atoms in the foil each next layer reflects the half of the signal back and in the end only very small fraction of wave's energy will pass through the foil (due to the great number of atoms in the foil cross section)? –  BartoNaz Dec 10 '12 at 11:14
    
@BartoNaz - the atoms absorb the EM wave but reemit it back in the same direction because of momentum. It's more like to a ball bouncing off a net of springs/rubber bands –  Martin Beckett Dec 10 '12 at 14:08
    
OK, I'll have to think about this more. Thank you for the answers. –  BartoNaz Dec 10 '12 at 16:02

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