Radiation

Radiation is energy moving in the form of light. All warm objects emit light -- people, buildings, and the earth. However, the light is not visible if the object is merely warm -- it is infrared light. Really hot objects emit visible light as well as invisible infrared light. This process is called

Truck and backhoe
This photograph was taken using infrared light; the different colors indicate different temperatures. Red and yellow places are warmer places, where more infrared light is being radiated. The engine and brakes of the truck, the hydraulic system of the backhoe, and even the truck driver are warmer than their surroundings. We thank John Fleckenstein of the Kentucky Department of Transportation for this picture

transfer of heat by radiation. You can feel the energy being radiated by a warm unit on an electric stove, by waving your hand over it. In most common applications, conduction and convection are more important than radiative heat transfer, but radiation can carry energy large distances quickly and doesn't require the presence of matter. Radiation moves readily through the vacuum of space as well as through air and many other materials.

Clear nights are cold nights because infrared light emitted by the ground can escape the earth, radiating out through the atmosphere and on into space. Clouds, however, can block the infrared light, reflecting it back to earth. The result is that cloudy nights are relatively warmer than clear nights.

The pattern of frost or dew on the ground is an indicator of how much different areas radiate. Exposed areas that have unblocked access to the night sky cool more rapidly than ones that cannot see the whole sky. Regions that are shaded in the day (no light coming in) remain warmer overnight (no light going out). So under a tree it is cooler during the day, and yet warmer at night.

White and silvery objects reflect light that hits them -- the light energy can't get inside of them. It turns out that this sort of object also don't radiate -- light energy can't get out.

The sun's light bounces off and is not absorbed by the suit

Astronauts wear silvery spacesuits to keep from getting broiled on their sunny side, and also to keep from radiating away all their energy and freezing on their shadowed side.

The astronaut is not cooled by radiation, because the infrared light can't get out

A special application of radiation is the use of microwaves to heat something. Microwaves are a kind of invisible light. Food is somewhat transparent to the microwaves, so the microwaves get well into the interior before being absorbed. The result is that the material is warmed throughout its volume.

With just one potato in the microwave oven, all the energy is absorbed uniformly within it, and the potato is quickly and uniformly cooked. In contrast, when baking a potato in a conventional oven, the outer surface of the potato absorbs energy from the hot air in the oven, and this energy then moves from the outside of the potato inwards by conduction. This is not a very fast process, especially because we can't let the outside of the potato get too hot (or it will burn). This is why it takes so long to bake a potato or roast a turkey. The microwave oven is the fast way to warm something up, but if the different materials absorb at different rates, we can get scalding hot cheese in a cool bun.

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Discussion of transport of energy by convection and radiation