Radiation through a Vacuum

It’s a beautiful day and you’re outside enjoying the warm sunshine. A few minutes later a cloud moves over the sun and you feel a chill. After the cloud moves on you once again feel warm.

It’s obvious that the sun transfers energy when you feel it on your face. When the sun is shining on you the energy of the sun warms you. But, it’s fairly common knowledge there are millions of miles of nothing, of vacuum, between us and the sun.

How does that energy get to us?

There are three methods of heat transfer between objects. One is conduction, that is when an object is in direct contact with another object and the heat is transferred. We see this when the handle on a pot on the stove gets hot. The heat is conducted through the metal of the pot to the handle, and further, to your hand if you’re careless. Another method of heat transfer is convection. Convection is the transfer of heat through moving fluids. When a breeze passes and we feel a chill that is from the movement of the air past our bodies transferring heat from us to the air.

But those two methods of heat transfer don’t tell us how the energy of the sun warms our face. There is obviously no contact between the sun and the earth, and the gases in interplantary space are far too thin to carry a significant amount of heat. To understand how heat travels through space we need to explore the third method of heat transfer, radiation.

Radiation brings up scary thoughts of nuclear events, and that is related. But the idea of radiation is a broader concept. Radiation is simply the transfer of energy through electromagnetic waves. Some electromagnetic waves are very high energy and can be dangerous to life, that’s where the scary stuff is. Radio is transmitted through electromagnetic radiation as well, however. And radio waves are fairly low energy and relatively harmless. Light is also an electromagnetic wave. In fact it is possible to consider all electromagnetic waves as a form of light, some we can see, which we call visible light, and some we can’t.

Heat is transferred through electromagnetic waves that are of a particular energy level. Visible light has an energy level that the structures in our eyes detects and then send signals to the brain. Radio waves are of an energy level that we can build antennas that detect those waves and convert the radio waves to movements of electrons that create electricity. When the radio waves are used in a particular way we can use that electricity to transmit information in the form of sound or television. Heat waves, or infrared light, is of an energy level that causes the molecules that they hit to increase their motion. That motion of molecules is what we call heat.

The sun is very hot and radiates, to one degree or another, almost all energy levels of electromagnetic waves. We know that it radiates visible light because that’s what makes day into night. And scientists can measure the other waves and use these to study the sun. We can feel the heat on our skin from the infrared, or heat, radiation.

In regards to the heat radiation, any time two objects are of a different temperature they will transfer heat from the warmer to the colder. The greater the temperature difference, the greater the heat transfer, until both end up at the same temperature. In the case of the sun it is very, very hot. It is always transferring heat out in every direction. Fortunately, for us, it is far enough away that we only get a small portion of the heat of the sun.

Another factor in how heat transfers through radiation has to do with the colors of objects. Color actually has to do with the energy of light that is reflected or absorbed. A black object absorbs a lot of the energy that hits it and reflects very little. A white object reflects a lot of energy and absorbs very little. This is why the metal of a dark car will feel hotter after it has sat in the sun than a white car. Scientists and engineers can calculate how much energy a material absorbs and determine the transfer of energy mathematically.

The transfer of energy is a very important study in science and engineering. There are people that specialize in just studying these phenomena. It’s not necessary to spend years at a university, however, to have a basic understanding of these concepts.