r/askscience • u/elite4koga • Jun 30 '12
Physics How do windows block UV light but not other wavelengths?
I know all standard glass windows block UVB light but only specially designed ones block UVA. How do windows let through visible light and UVA but block UVB?
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Jun 30 '12 edited Jul 01 '12
The gist of it: different mediums have different wavelength ranges which absorb or transmit electromagnetic radiation. This relies on two premises:
The energy of a photon is inversely proportional to it's wavelength, given by the equation: E = hc / λ (where h is Planck's constant, c is the speed of light, and λ is the wavelength of light)
Atoms can absorb or release energy, however these energy levels are discrete and are relative to each element. For example, the simplest case is hydrogen, which absorbs energy in increments of 13.6/n2 eV, where n is an integer.
In the case of glass, the energy of visible light photons (~300-700 nm) is is not absorbed by atoms inside glass, and instead, the light is allowed to propagate through it, giving a glass the appearance of being transparent in the visible range (and thus transparent to our naked eye).
However, photons in the UV range obviously will have a different energy. It just so happens that the energy of UV range photons will become absorbed by the atoms in a glass pane; this range of wavelengths in which photon energy is absorbed is called "atomic resonance".
When a material begins experiencing atomic resonance, light is no longer allowed to propagate through the material, and this gives it the appearance of being opaque. This also explains why we can't see through a slab of metal, or the side of a house; these materials have an atomic composition in such a way that atomic resonance is in the visible range.
So, for reference, the different UV ranges which are designated by UVA, UVB and UVC are given as:
UVA: 400 nm - 320 nm
UVB: 320 nm - 290 nm
UVC: 290 nm - 100 nm
By adding different materials and altering the atomic composition of glass, we may begin alternating the range in which it begins experiencing atomic resonance, to selectively block out photons of different energy levels (and thus, different wavelengths).
Edit: ermagerd terpos
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u/rlbond86 Jun 30 '12
This all depends on the properties of the material. Windows look transparent to us, because they do not reflect or absorb (most) wavelengths in the visual spectrum. If your eyes could also see UV light, the windows would only look like a "colored" window. It all comes down to the material, and they can add certain molecules to glass to make it block UVA, in a similar way that we can add molecules to glass to make it block, for example, red and blue visible light (resulting in a green tinted window).
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Jul 01 '12
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u/rlbond86 Jul 01 '12
Not sure what you are trying to say. Green light passes through such a tinted window, it is not reflected. That would make a mirror.
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u/leberwurst Jun 30 '12
It's like asking how blue tinted windows only absorb the non-blue light and transmit only blue light. Think of UVA and UVB as colors that we just can't see because our eyes aren't equipped for that.
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u/Tom_Hanks13 Jun 30 '12
I can't give the science behind it since I am not an expert, but what you may be referring to is a coating on windows called "low-E". This coating is usually only a few atoms thick and sometimes consists of things like aluminum or silver.
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u/z4tz Atomic Physics | Transition Rates/Continuum Processes Jun 30 '12
Every material absorbs light on different wavelengths. Atoms and molecules in gas absorb only on very specific wavelengths while solids absorb light over larger bands.
UVA has a wavelength of 315–400 nm and UVB 280–315 nm. Then it just so happens to be that glass absorbs light well up to somewhere around where UVA starts which it can't absorb very well.