# How does light filtration work?

How exactly do certain types of glass filter out light within specific frequencies/wavelengths?

I'm wondering because I read somewhere that certain types of windows filter out certain parts of the electromagnetic spectrum. I've heard about polarization of light, but don't understand how this would apply here or even work in type of glass used in windows.

So in other words, I'm asking: what happens to the light waves as they pass through different types of glass, besides simply being refracted and reflected? How can certain portions of the spectrum just be removed? In case there are multiple methods/effects, please either elaborate or enumerate.

• It depends on the technology. We can make wavelength selective mirrors which work by interference on thin layers. That's very expensive and usually only being used for scientific/engineering applications. There are thin metal layers that act as partially reflective mirrors and then there are absorptive glasses that get hot because they convert the light that they filter out into heat. The latter is the least useful but probably cheapest approach. The main problems with it are the heat they glass gives off and the risk of thermal expansion leading to cracks and dangerous tension in the glass. – CuriousOne Jun 7 '15 at 8:28
• How does the technology work? Is it like sparsely fitting micro chips into a layer of the glass and filtering out unwanted light with the help of some sort of lasers or vibrations? Are certain glasses designed using a special method of preparation that causes them to reflect away certain parts of the spectrum and still remain transparent? What makes the absorptive layer able to absorb heat? I like the idea but can you please elaborate? I know it may be a lot to ask, but I really need to know the details of how special glasses work. – tkhanna42 Jun 7 '15 at 11:23
• How do the mirrors not affect the transparency of the glass? – tkhanna42 Jun 7 '15 at 11:24

The low energy tail, which is below the bandgap of the material, can for example be absorbed through the excitation of lattice vibrations, so called phonons, in the material. $SiO_2$ e.g. has phonon modes around 150 and 60 meV, which absorb some fraction of infrared light.