# Do hot objects always glow?

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## Main Question or Discussion Point

I'm talking hotter than T = 6000k.
The higher the temperature, the more the curve in the attached figure would shift to the left (while at the same time getting higher).

So the intensity peak would eventually fall back into the invisible portion (very small wavelength this time) of the electromagnetic spectrum. (UV and beyond)

You would still see visible light though, right? Just as you could still detect infrared radiation.
Is this always the case? Or can an object be so hot that it just doesn't radiate any visible light?

Me, personally, I don't think so. But I figured it'd be an interesting question to ask. Tried googling it. Couldn't find an answer relatively quickly.

-Yael

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Buzz Bloom
Gold Member
Hi APUGYael:

Do you see in the chart that for higher temperatures, a greater amount of power occurs in the visual part of the spectrum than for lower temperatures. The fact that the peak of the radiation is outside of the visual range, does not change this.

Regards,
Buzz

Hi APUGYael:

Do you see in the chart that for higher temperatures, a greater amount of power occurs in the visual part of the spectrum than for lower temperatures. The fact that the peak of the radiation is outside of the visual range, does not change this.

Regards,
Buzz
Exactly what I figured. Sorry, I might've been unclear. What I am asking is if the curve radically changes behaviour at a certain (high) temperature.

kuruman
Homework Helper
Gold Member
Here is an illustration of what @Buzz Bloom is saying. The vertical grey lines mark the visible range and the temperature is T = 20,000 K.

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Drakkith
Staff Emeritus
Exactly what I figured. Sorry, I might've been unclear. What I am asking is if the curve radically changes behaviour at a certain (high) temperature.
No, the behavior remains consistent for any temperature. The hotter the object becomes, the brighter it glows in the visible range.

Here is an illustration of what @Buzz Bloom is saying. The vertical grey lines mark the visible range and the temperature is T = 20,000 K.

View attachment 224083
Yeah, I know what you're/he is saying. That's what I was talking about in my original post.
"The higher the temperature, the more the curve in the attached figure would shift to the left (while at the same time getting higher)."
i.e. the peak shifts up and to the left, so that all forms of electromagnetic radiation also become more intense (read: also move up)

I'm asking if there's a certain temperate where this behaviour is no longer valid.

Buzz Bloom
Gold Member
I'm asking if there's a certain temperate where this behaviour is no longer valid.
Hi APUGYael:

Answer to the question quoted above.
NO.

Regards,
Buzz

No, the behavior remains consistent for any temperature. The hotter the object becomes, the brighter it glows in the visible range.
Would very hot objects be white or blue?

White would be understandable because all wavelengths of visible light are emitted, but blue would always have higher intensity on the chart (for high temperatures).
Would objects then look blue, but looking at them would look like looking at the sun (white light entering eye)?

Drakkith
Staff Emeritus
Would very hot objects be white or blue?
According to a spectral radiation calculator I found, at 100,000 K there is roughly 5x more power in the blueish end of the visible spectrum than in the red end. This is also the case for 50,000 K and 10,000 K. I assume the pattern holds for temperatures higher than 100,000 K, but the calculator does not go that high. This would probably make very hot objects look bluish-white unless you were close enough that your eyes are saturated at all wavelengths, in which case it would just look blindingly white.

OmCheeto
Gold Member
According to a spectral radiation calculator I found, at 100,000 K there is roughly 5x more power in the blueish end of the visible spectrum than in the red end. This is also the case for 50,000 K and 10,000 K. I assume the pattern holds for temperatures higher than 100,000 K, but the calculator does not go that high. This would probably make very hot objects look bluish-white unless you were close enough that your eyes are saturated at all wavelengths, in which case it would just look blindingly white.
I did the calculations up to 6 trillion Kelvin, and the pattern holds for higher temperatures.

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