Transferring Heat through Radiation

Radiation is one way to transfer heat. You experience radiation personally whenever you get out of the shower soaking wet in the dead of winter and bask in the warmth of the heat lamp in your bathroom. Why? Because of a little physics, of course. The heat lamp, which you see in the figure, beams out heat to you and keeps you warm through radiation.

An incandescent light bulb radiates heat into its environment.
An incandescent light bulb radiates heat into its environment.

With radiation, electromagnetic waves carry the energy. Electromagnetic radiation comes from accelerating electric charges. On a molecular level, that’s what happens as objects warm up — their molecules vibrate harder and harder, causing acceleration of electric charges.

Heat energy transferred through radiation is as familiar as the light of day; in fact, it is the light of day. The sun is a huge thermal reactor about 93 million miles away in space, and neither conduction nor convection can produce any of the energy that arrives to Earth through the vacuum of space. The sun’s energy gets to the Earth through radiation, which you can confirm on a sunny day just by standing outside and letting the sun’s rays warm your face.

Every object around you is continually radiating, unless its temperature is at absolute zero (which is a little unlikely because you can’t physically get to a temperature of absolute zero, with no molecular movement). A scoop of ice cream, for example, radiates. Even you radiate all the time, but that radiation isn’t visible as light because it’s in the infrared part of the spectrum. However, that light is visible to infrared scopes, as you’ve probably seen in the movies or on television.

You radiate heat in all directions all the time, and everything in your environment radiates heat back to you. When you have the same temperature as your surroundings, you radiate as fast and as much to your environment as it does to you. When two things are in thermal contact but no thermal energy is exchanged between them, they’re in thermal equilibrium. If two things are in thermal equilibrium, they have the same temperature.

If your environment didn’t radiate heat back to you, you’d freeze, which is why space is considered so “cold.” There’s nothing cold to touch in space, and you don’t lose heat through conduction or convection. Rather, the environment doesn’t radiate back at you, which means that the heat you radiate away is lost. You can freeze very fast from the lost heat.

When an object heats up to about 1,000 kelvins, it starts to glow red (which may explain why, even though you’re radiating, you don’t glow red in the visible light spectrum). As the object gets hotter, its radiation moves up in the spectrum through orange, yellow, and so on up to white hot at somewhere around 1,700 K (about 2,600 degrees Fahrenheit).

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