How Everything Works
How Everything Works How Everything Works

Question 603

What is the relationship between dark material and heat? Why does dark material absorb more heat than light material? - AR
Thermal radiation consists of electromagnetic waves. These waves are emitted and absorbed by the movements of electrically charged particles, usually electrons. Since all materials contain electrically charged particles, any of them can interact with thermal radiation. However, these interactions differ from material to material. The electrons in some materials are extremely effective at absorbing and emitting thermal radiation and these materials appear black. When the sun's thermal radiation strikes a black material, that material absorbs the sunlight and nothing reflects. That's why the material appears black. When you heat a black material to high temperatures, it also emits thermal radiation extremely well—for example, a hot piece of black charcoal glows brightly with its own red thermal radiation.

Materials in which the electrons are not able to absorb or emit thermal radiation have one of several familiar characteristics. Some are clear, meaning that thermal radiation passes right through them. Others are white, meaning that thermal radiation that strikes them is scattered uniformly in all directions. Still others are mirror-like, meaning that thermal radiation that strikes them is reflected in specific directions. All of these materials are virtually unable to emit their own thermal radiation: clear glass, white sand, and mirror-like aluminum emit very little thermal radiation even when they're "red hot."

Since black objects are best at emitting and absorbing thermal radiation, they are best at transferring heat via radiation. A black object will receive more heat from the hotter sun than a white object of similar dimensions and temperature. A black object will also radiate more heat to its colder environment than a white object of similar dimensions and temperature, although here "black" and "white" refer to the object's behavior regarding its own thermal radiation. Near room temperature, thermal radiation is in the infrared, and many objects that appear white to visible light are actually rather black to infrared light.


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