Blackbodies, Temperature and Color

Understanding blackbodies and the relationship between their temperature and color allows us to study stars and other objects in the universe. Below, you'll find an explanation of blackbodies and the relationship between their temperature and their color.

Reflection vs. Emission

In class, Profesor Kirshner put 2 objects that looked very different at room temperature into a (very hot) ceramics oven, and heated them up until they appeared to be the same color.
Why, at room temperature, did they have different colors, but when hot enough, they looked the same?
The answer in short is, "reflection vs. emission".
At room temperature, we saw the objects by the light that they reflected. In other words, if we turned off all lights in the room, there would be no light for the objects to reflect, and so we would no longer see them.
After heating up in the oven, however, the objects were both hot enough glow, that is emit light, at wavelengths that our eyes can detect. Even with the room lights off you would see the objects glowing. They were making their own light.
Now, why wouldn't the objects at room temperature glow? They do! All objects emit radiation. You and I do. The walls do. Your dog does. But in order for the emitted light to be seen (that is, detected by our eyes) the object has to be hot enough.
If we had infrared goggles, we would be able to see objects at room temperature by their emitted light. This is the source of the name "night vision goggles." At night, the sun is down, and (we assume) there are no other lights around. So it's like being in a room and turning off the lights. The room appears pitch black to our eyes, but if you had eyes (or goggles) that could see infrared wavelengths, then you would be able to see people by their body heat.
Figure 5-9 at right (and in the textbook) shows what a boy and his dog look like in infrared light. In the picture, white is warmest and black is coolest.
We can see that the boys face is warmer than his clothes, and that the dog's nose is very cold, which matches our everyday experience (at least for those who have touched a dog's nose).

A Blackbody

A blackbody is an object that emits a well defined spectrum of radiation solely based on its temperature. We see from figure 5-10 at right that the hotter the blackbody, the more intense it is, and the shorter the peak wavelength.
The picture does not say anything about what the object is made of, or how heavy it is, etc. It doesn't matter! The only property that determines the spectrum of a blackbody is its temperature. Brick, iron or a dense gas will emit the same spectrum as long as they are at the same temperature. That spectrum will have a peak that lies at a particular wavelength, lambda_max.

Color and Temperature

The peak wavelength corresponds to a color. In Figure 5-10 at right, we see that at the temperature increases, the spectrum peaks at smaller and smaller wavelengths. At T=3000K, the peak is outside of the visible. In fact it lies in the infrared. But you still see some intensity in the visible, most of which lies in the red wavelengths. So the object will appear reddish.
The T=12,000K object, however, peaks at a wavelength shorter than the violet (it peaks in the ultraviolet). Still, there is emission in the visible. In particular, the emission in the blue/violet dominates. This object would appear a bluish-white.