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Q & A: White Hot or Blue Hot

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Q:
Which is hotter white fire or blue fire?
- Angela McCullough (age 14)
Yuma,AZ,USA
A:
Angela,
This is a great question. As you suggest, the color of an object can tell us something about its temperature.

It turns out that an object which is at a fixed temperature will give off light over quite a broad range of wavelengths. The center, or average, of these wavelengths are related to the temperature. If you heat a piece of steel until it looks white, it is quite hot. The wavelengths of the light that this piece of steel gives off range from red to blue, which (when mixed together) look white to our eyes.

If something is even hotter than this (so hot that it looks blue to our eyes) it means that the wavelengths of the light that this object gives off probably range from blue up through the ultra-violet (UV). Since our eyes cant see UV light, it just looks blue to us.

So, unless something else is going on, an object that looks blue probably has a higher temperature than something that looks white.

Having said this, you should realize that this may not always be true. Some chemicals burn with a blue color, for example, so that if you burn some of these on an ordinary fire it will look blue for a while (some fireplace logs may do this). This does not mean that the temperature of the whole fire went up, just that these chemicals made the color change.

Mats

(republished on 07/24/06)

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