Glass has low transmittance for infrared light. Does glass reflect or absorb infrared light? Greenhouses are able to reflect infrared as I understand it.

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In fact it depends on the type of glass. Depicted is solar transmittance through glas. For greenhouses special heat absorbing glas may be used. The diagram shows a dip of transmittance at $\approx 1200\,$nm. Lower transmittance is because absorption (as in the name of the glas) and as well in reflection (see Fresnel equations). Your reasoning is right: the glas of a greenhouse may reflect infrared light.

Transmittance of glass

The most common types have a transmission band up to the NIR (near infrared) regime. E.g. crown glas ($350\,\text{nm}<\lambda <2\,\mu$m) and Fused quartz ($200\,\text{nm}<\lambda <3\,\mu$m).

It does both. It reflects some infrared light because in the IR glass has a different refractive index than air. The percentage of IR light that enters the glass is mostly being absorbed (depending on glass thickness of course).

Gases have a more complicated absorption spectrum and almost all gases have absorption bands somewhere in the (mid) IR, where they absorb light. There is almost no reflection, because gases all have a refractive index close to 1.

The greenhouse effect is due to absorption of thermal radiation from earth and not from reflection. But either way, the result (i.e. that heat is not radiated to space) would be the same.

  • I understand that glass greenhouses work simply by stopping winds or convection bearing heat away from their insides: the Earth's or a planet's is something quite different and the result of absorption and reemission of IR by greenhouse gasses. – WetSavannaAnimal aka Rod Vance Oct 4 '13 at 1:36
  • Yes, I was talking about the greenhouse effect on our planet, not on actual greenhouses (there you are right of course). Maybe I have misinterpreted the original question. – Andreas H. Oct 4 '13 at 2:48

For optical glass, they provide the excellent transmittance (visible wave) range about 400-800 nm. Of course you have several types of glass. Glass with low refractive index has high transmittance. Greenhouse can reflect the infrared because the wavelength of the infrared is longer compared to the glass that used for greenhouses to absorb shorter wavelength (visible) and have the high refractive index. The ability for glass to absorb or reflect depends on the type of the glass. The higher the refractive index, the the more capable to absorb infrared light.

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