So I understand that matter emits EM waves when hot. And that the higher the temperature, the shorter the wavelength, so cooler flames start off orange and the hotter flames reach light blue and white. What I'm wondering is if, hypothetically, you could take that blue-white flame and could make it considerably hotter, would the emitted wavelength reach UV thus making it invisible to the human eye? If so what temperature would need to be reached?

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    $\begingroup$ No, when something gets hotter it emits more at every wavelength. When it gets super hot, most of the power will be coming out in UV but there will still be plenty in the visible spectrum. $\endgroup$
    – knzhou
    Sep 7 at 19:35
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    $\begingroup$ @knzhou sounds like an answer to me $\endgroup$ Sep 7 at 19:39
  • $\begingroup$ @knzhou Agree with BioPhysicist. $\endgroup$
    – Bob D
    Sep 7 at 21:34
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    $\begingroup$ Ethan, everything with a temperature (meaning EVERYTHING) radiates electromagnetic energy. Cold objects radiate in the infrared and hot objects radiate in the visible spectrum. $\endgroup$ Sep 7 at 21:41

1 Answer 1


According to Wien's displacement law, for rising temperatures the maximum of emitted radiation shifts from infrared to visible, ultaviolet, X-rays, ...

But the maximum of perceived light shifts from red, orange, yellow, white to bluish-white and then stays there even for infinitely high temperature (see the point marked with $\infty$ in the image below).

enter image description here
(image from Color temperature)

  • $\begingroup$ OK that makes sense, but your answer raises another question for me, why does emitted light plateau at the light-blueish region and not continue into ultraviolet? $\endgroup$
    – Ethan
    Sep 7 at 20:16
  • $\begingroup$ @Ethan Well, the perceived maximum needs to be in the visible spectrum. It cannot be in the ultraviolet simply because we cannot see ultraviolet. $\endgroup$ Sep 7 at 20:20
  • $\begingroup$ But why does it need to be in the visible spectrum? Visible light is an arbitrary range from a physics perspective, it's just the most abundant wavelength on earth and so it what pur eyes evolved to detect. Why does incandescent light stay within that spectrum? $\endgroup$
    – Ethan
    Sep 7 at 21:00
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    $\begingroup$ So essentially the simple answer is, it will start to radiate UV and other invisible higher energy wavelengths, but it won't stop radiating the visible wavelengths and so will appear to stop at a blueish glow? $\endgroup$
    – Ethan
    Sep 7 at 22:13
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    $\begingroup$ @Ethan Yes, that's it. $\endgroup$ Sep 7 at 22:19

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