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After refraction in a prism white light disperses to VIBGYOR colored radiation and made to fall on a flat screen. Are regions immediately below violet rich in ultra-violet radiation and those above red have more infra-red radiation? If so, has any heating been noticed in these edge areas by sensitive measurement?

That is, does white light have absolutely no UV or IR radiation at all in the spectrum?

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  • $\begingroup$ What do you mean by "white light"? How is your white light produced? $\endgroup$ Jan 17 '18 at 15:45
  • $\begingroup$ From the sun intensified if required by any means. $\endgroup$
    – Narasimham
    Jan 17 '18 at 15:53
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You are describing experiments done by Herschel in the early 1800s. He used a prism to disperse sunlight and put a thermometer under each color and by either accident or through insight decided to go "beyond red". Ultraviolet will be similar.

Prisms spread white light into different colors by a process called dispersion: The prism bends higher frequencies / shorter wavelengths (the blue and ultraviolet end of the spectrum) more than low frequencies / long wavelengths (the red and infrared region of the spectrum). The amount of bending depends on the index of refraction $n$. Here's what these look like for around visible light in a quartz prism: from a NIST document Taken from this NIST publication. Visible light is from 0.4 to 0.7 microns in wavelength, so you can see these graphs extend to ~0.2 (ultraviolet) and +1.0 (infrared) microns of wavelength. It is also important to note this doesn't extend forever. The parts of the EM spectrum that bracket the visible range are called the near infrared or the near ultraviolet. Beyond those are the far infrared and the far ultraviolet, and they will no longer disperse in a quartz prism.

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Sunlight contains plenty of infrared and UV light, and this light is refracted through a prism just like visible light*. The infrared light ends up below red, and the ultraviolet light ends up above violet (hence the names). The infrared light does indeed manifest as heat - in fact, this is how infrared light was originally discovered. Someone held a thermometer at various places in the prism image and noticed that it was heating up below red, where there was no visible light.

*There's a small caveat here: prism materials may absorb infrared and ultraviolet light, or the refractive index for those bands may be substantially different than for the visible band. In either case, the above will be less accurate.

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  • $\begingroup$ What I mean is.. how is energy distributed across frequency/wavelength without prism material attenuation ? $\endgroup$
    – Narasimham
    Jan 17 '18 at 16:09
  • $\begingroup$ The solar irradiance spectrum answers that question. Here's a plot of it: en.wikipedia.org/wiki/Sunlight#/media/… $\endgroup$ Jan 17 '18 at 16:11

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