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when william herschel conducted the experiment of separating white light with a prism and measuring the different colors, he put a thermometer past the red color as a control finding it to pick up the most heat, thus concluding infra red.

my problem is... Infra red doesn't pass through glass... so how did he pick it up from the prism? was it not glass (i think it was), did it slow the wave down to make it into infra red, or does very near wave infra red pass through it differently? please can someone shed some light !

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You seem to be assuming a glass prism, which is not stupid, but neither is it guaranteed. In particular quartz is fairly transparent in a large swath of the IR. (Also note that "IR" is a wide band and many materials can have very different properties at different IR wavelengths.) –  dmckee Apr 3 '13 at 14:04
    
i thought this may be the case. yes, i know they used rock salt (Nacl) after for IR as it was very good optically for thermal measurements ect. but was not sure about what he used. i have read glass but this could of been wrong. –  Chris Deakin Apr 3 '13 at 14:09
    
many places say it was glass or don't mention the material. i have also read that he got the prism from a Chandelier (i don't know if that precludes quarts) –  Chris Deakin Apr 3 '13 at 14:12
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Even common glass has some transmission in IR, especially in NIR. Look at the transmission curve here. –  jkej Apr 3 '13 at 14:13
    
ahh thank you :) so its a NIR thing, i normally detect IR at longer wave lengths, thus not seeing the same transmission. –  Chris Deakin Apr 3 '13 at 14:22

1 Answer 1

The description infra-red covers a wide range of wavelengths from wavelengths of about 700nm out to tens of microns. Ordinary glass will transmit radiation out to at least 1 micron - the exact cutoff depends on the type of glass.

In the prism experiment the geometry means that that the infra-red light detected can't have a wavelength more than (very roughly) twice the longest visible light so you're talking about wavelengths shorter than around 1.5 microns. This near infra-red will pass through a glass prism so it can be detected by a thermometer.

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