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Q & A: So hot you can't see it?

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Most recent answer: 12/09/2014
As an object gets hotter, the emitted radiation moves from infrared to visible spectrum giving an orange-red glow to it when it has reached few thousand of degrees. If it gets even hotter, its glow change from red to white as the emitted radiation continues to shift towards the blue end of the spectrum. At few tens of thousands of degrees, the shift in the emitted radiation is such that objects start to glow bluish. My question is as increasing the temperature shifts the emitted spectrum towards UV and Xrays range, could an object be so hot (say quadrillions of degrees) that it ends up "glowing" black since all of the emitted radiation is now outside our visible range (Gamma rays)? Thank you
- Anonymous

As you described, the peak wavelength emitted by a hot object (or any object above absolute zero) gets shorter as its temperature increases. This shift is described by , which says that the peak wavelength is inversely proportional to temperature. While the emission is strongest around the peak wavelength, in theory the object actually emits some light at all wavelengths, from x-rays to the infrared. The plot below shows some example thermal radiation spectra for different temperatures.

So, you asked whether it's possible to increase the temperature so much that all the light is emitted in wavelengths we can't see. As the temperature of an object increases, the amount of visible light that it radiates will actually always increase. As the temperature gets higher and the peak wavelength gets shorter, the object also just gets brighter—emitting more light at all wavelengths, including the visible part of the spectrum. You can see this in the plot: the lines representing higher temperatures are above the cooler ones over the whole range of wavelengths. 

It took scientists a long time to figure out why these spectra have the shape they do. Classical theory why there's a peak in the spectrum, instead of just more and more energy at shorter wavelengths. Requiring light to be emitted and absorbed in discrete "packets" instead of a classical continuum turned out to solve the problem mathematically (although was thinking about a different problem when he first proposed this idea). It wasn't until even later that proposed that these mathematical quanta were real particles, what we now call photons. 

Rebecca Holmes

(published on 12/09/2014)

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