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The word “radiation” fills many of us with fear, while the word “light” is associated with cheer and hope. Yet, from a scientific standpoint, the two concepts are tightly intertwined. Furthermore, we are surrounded by more kinds of radiation — and more kinds of light — than most people realize.
Every moment of every day, you are constantly bombarded by radiation. In fact, every object around you constantly emits radiation. Even your own body emits radiation. This may sound scary, but most of this radiation is harmless. Once you understand this constant exchange of radiation — and once you understand what happens when that radiation is emitted or absorbed — it will explain a lot of little mysteries that we encounter in the world around us. The main hurdle is getting past our preconceptions related to the words “radiation” and “light”. When you think about radiation and light in a slightly different way, it opens a new window for viewing the world, allowing you to see and understand phenomena that surround us every day.
Radiation
So let’s start with the word “radiation”. Most people associate the term with nuclear reactors and radioactive waste. In this interpretation, radiation is a dangerous thing. You certainly don’t want to be exposed to it, and you would be terrified to discover that there is any in your house. However, this popular use of “radiation” as a synonym for “nuclear radiation” is extremely narrow, because it excludes all sorts of radiation that don’t originate from the radioactive decay of atomic nuclei.
Before we look at the broader definition of “radiation”, let’s take a closer look at nuclear radiation. You have probably heard of early 20th century scientific pioneers, such as Pierre and Marie Curie, who were fascinated with the newly discovered phenomenon of radioactivity. These people realized that certain substances, such as the element radium, emitted something invisible that could affect other materials nearby — for example, causing unexposed photographic film to fog up. This mysterious phenomenon was called radiation, because it appeared to radiate in straight lines in all directions from the source substance.
Subsequent experiments soon revealed that radioactive materials emit three distinct kinds of radiation, which have vastly different abilities to penetrate other materials. It was not yet understood what these three kinds of radiation actually were, so they were simply designated by the first three letters of the Greek alphabet — alpha rays, beta rays, and gamma rays. It was eventually determined that all three types of radiation resulted from nuclear decay. In other words, certain kinds of atoms have an unstable nucleus — and when the nucleus decays, transforming the atom into a different kind of atom, then radiation is emitted. Alpha rays and beta rays are particles of matter ejected at high speed, while gamma rays are very powerful bits of energy.
Of course, we now know something that the early researchers did not know — that nuclear radiation is dangerous to living cells. Of particular consequence is the damage that nuclear radiation can cause to DNA, because this can lead to cancer and to birth defects.
However, another key takeaway is that the word “radiation” turned out to mean two different things — 1) ejecting subatomic particles at high speeds, or 2) emitting pure energy. Further examination of gamma rays revealed that they travel at the speed of light, which, as you may know, only light can do. In fact, gamma rays are indeed a form of light. The difference between gamma rays and visible light is that gamma rays have a much shorter wavelength. But otherwise, gamma rays and visible light are identical.
The Many Kinds of Light
There are many kinds of rays that are just like gamma rays or visible light, except that they have other wavelengths. This includes microwaves, X-rays, and radio waves. It also includes ultraviolet light and infrared. All of these are therefore simply different forms of light. The sun emits all of them, and they all travel from the sun to the earth at the speed of light — although not all of them are capable of penetrating the earth’s atmosphere. The upshot is that the word “light” has two different meanings. In common speech, we typically use the word to mean only visible light — which of course is a very human-centric way of defining the term. (Many kinds of animals, including birds and bees, can see forms of light that can humans cannot see.) In contrast to everyday language, scientists tend to use the word “light” in a much broader sense, referring to all wavelengths of light, whether visible to people or not.
To avoid this ambiguity, scientists often use the more precise term electromagnetic radiation to mean any kind of light, regardless of the wavelength. However, this brings us back to the word “radiation”. This term tells us that all forms of light, not just gamma rays, are in fact radiation. This means that every object that produces visible light, such as a light bulb, emits radiation. Any device that radiates heat, such as a space heater, sends out infrared radiation. Any device that sends messages wirelessly is putting out radio waves. Your microwave oven generates microwaves, although in this case the radiation remains enclosed within the walls of the device. Even the word “radio” comes from the same root as “radiation”, because radios and cell phones work by using electromagnetic radiation.
We certainly don’t worry about the radiation that comes from a light bulb, because we have no fear of visible light. Most of us have no fear of the radio waves that come from distant radio and TV stations — even though you and everything around you is bombarded daily by the waves from several such stations. But we know that living things can be harmed by some kinds of electromagnetic radiation, such as X-rays or gamma rays. And therefore we tend to fear all of the less familiar forms of light, because we are unsure whether to consider them safe or harmless.
It can be quite helpful to think of all forms of light as being part of a continuous spectrum. Just as a rainbow includes all wavelengths of visible light, the full spectrum of electromagnetic radiation encompasses all forms of light. This spectrum continues on beyond red at one end of the rainbow, and on beyond violet at the other end of the rainbow. As you go beyond red, through infrared and on to microwaves and radio waves, the wavelengths get longer. As you go beyond violet, through ultraviolet and on to X-rays and gamma rays, the wavelengths get shorter. Here is a diagram that illustrates this spectrum:
Now here is a point that may seem counter-intuitive. The longer the wavelength, the less energy the light has. Shorter wavelengths have more energy. This may seem odd, because we think of microwaves and perhaps infrared as having a lot of energy, when in fact they have less energy than visible light. Therefore it is the other end of the spectrum, starting in the middle of the ultraviolet range, where the various forms of light have enough energy to damage our cells. As the wavelengths get shorter and shorter, the radiation gets more and more dangerous to living cells. As you can see from the diagram, gamma waves have extremely short wavelengths, and hence are particularly dangerous to us.
Another term you may sometimes see is ionizing radiation. This term refers to any kind of radiation that is powerful enough to knock electrons out of atoms, and therefore which can lead to chemical changes in substances. It is ionizing radiation that presents a danger to living cells, especially when the radiation affects the DNA in the cells. However, the term “ionizing radiation” refers not only to the dangerous wavelengths of light, but also to any beams of subatomic particles that have similar effects. Therefore all three classes of nuclear radiation — alpha rays, beta rays, and gamma rays — can be considered as ionizing radiation.
At this point in the discussion, we have now cast the terms “radiation” and “light” into a new light, so to speak. All forms of light are types of radiation, radiating out in straight lines from a source. But radiation is a broader term, because it also encompasses streams of invisible, subatomic particles — which are not forms of light. We are surrounded by radiation every moment of our lives, but very little of it has anything to do with nuclear radiation, and only a very small part of it is ionizing radiation.
Thermal Radiation
So now we are ready to talk about the radiation that surrounds us every day. Suppose you were to examine a large white chunk of rock. Why is it white? Because it reflects most or all of the visible light that strikes it. Suppose you were to examine a large black chunk of rock. Why is it black? Because it absorbs most or all of the visible light that strikes it. If you were to measure all of the electromagnetic radiation (light) coming from the two rocks, graphed according to the wavelength, you would record a big difference in the amount of visible light coming from the two rocks — because of the difference in reflectivity. But when you check the longer wavelengths in the spectrum, you find a region where both rocks give off nearly identical amounts of radiation. These emissions are tied to the concept of black-body radiation, and as the name indicates, it is completely independent of any reflected light. It turns out that everything that is warmer than absolute zero (-460 F. or -273 C.) — in other words, everything in the world around us — constantly gives off this radiation. Furthermore, the amount of radiation given off, and the wavelengths of that radiation, are both determined by the temperature of the object. A warmer object gives off a lot more radiation than a cooler object, and the wavelengths of the warmer object are shorter. Because of this strong connection to temperature, this kind of radiation is also called thermal radiation, which is the term we’ll use in the remainder of this discussion.
Even though the terms “black-body radiation” and “thermal radiation” refer to exactly the same phenomenon — radiation that is emitted solely due to the temperature of an object — the two terms have different meanings in scientific discussions. Black-body radiation is a theoretical construct, based on a hypothetical situation. If an object were perfectly black, absorbing all wavelengths of light that fell upon it (not just the visible light), and if that object met certain other criteria (such as being at a constant temperature), then we could calculate the exact amount of energy that the object would radiate, along with the distribution of wavelengths that would be emitted. This theoretical result is formally called black-body radiation. On the other hand, when we measure the actual energy given off by a real object as a result of its temperature, we call it thermal radiation. There are many situations where the actual thermal radiation of an object is quite similar to the theoretical black-body radiation.
In order to understand the profound effects of thermal radiation on the objects around us, it is helpful to recognize that in any given situation, there may be several distinct methods of heat exchange operating simultaneously. Imagine that you are sitting at your breakfast table, sipping a cup of coffee. You are wearing a light sweater because the room is a little bit cool. Your cup of coffee is the warmest thing in the room, and you are well aware that if you left it alone for a half hour, then it would cool off to room temperature. Why is that? One reason is conduction. The mug and the surface of the coffee are in contact with the air, and some of the heat goes into the air through direct contact. Another reason is convection. The air is slowly circulating around the room, and therefore the air that is warmed by the coffee is quickly carried away, replaced by unheated air. If it were not for convection, then the layer of air around the coffee mug would soon heat up and begin to act as thermal insulation. In fact, that is the main reason that you are wearing a sweater — to trap the air next to your body, preventing convection, and thereby providing a layer of insulation.
The third reason that your coffee is cooling is that some of it is evaporating, which causes a loss of heat, because energy is required to turn liquid water into a gas. But the fourth reason for the cooling is that your cup of coffee is giving off thermal radiation at a faster rate than anything else in the room. Because this radiation is a form of energy, any radiating object must be losing energy, causing the object to cool. On the other hand, if the thermal radiation strikes other objects nearby, then those objects are likely to absorb the radiation, thereby gaining energy and becoming warmer. If everything in the room is the same temperature, then the energy lost and gained through thermal radiation would be in balance. Conduction and convection would have no effect either, although there could still be some cooling from evaporation.
On the Ground, Under the Sky
Imagine now that you are no longer sitting in your kitchen, but instead you are sitting in an open, mowed field during a cloudless night. The air is starting to get rather chilly, much cooler than it was during the day. You happen to have a thermometer with you (how handy!), so you measure the air temperature at various heights above the ground, and you find that the coldest air is right next to the ground. “Well,” you say, “that makes sense, because cooler air tends to sink.” But then you measure the temperature of a rock sitting on the ground, and you discover that it is even colder than the air next to it. In fact, everything on the ground around you is colder than the air next to it. How could that be? Doesn’t the ground become cold at night because of contact with the cold air? But if that were true, then the ground would not be colder than the air.
And then you realize what is going on. The surface of the ground, and all of the objects sitting on the surface of the ground, are radiating thermal energy upward into outer space, but they aren’t getting much thermal radiation in return. Therefore everything on the surface of the ground is getting colder and colder all night long. The ground is not cooling due to contact with cold air — in fact, it is exactly the other way around. The air is getting colder due to its contact with the cold ground, which explains the temperature profile of the air — coldest right next to the ground. (In the daytime, when the sun is shining, the hottest air is right next to the ground, acquiring its heat by touching the solar-heated surfaces.)
A few weeks later, early on an autumn morning, you return to the same field at dawn. You see that the entire field is covered in a light frost. So you measure the air temperature five feet above the ground, which the standard height for measuring air temperature. Your thermometer says 35 degrees F. But how could that be? The freezing point of water is 32 degrees F. Does frost have a higher freezing point than other kinds of ice? But then you remember your experience a few weeks earlier, and you measure the surface of ground: 31 degrees F. Okay, that solves that mystery — the ground is colder than the air, cold enough for frost to form. Then you notice the trees that surround the field, and you see that there is no frost underneath the trees, just a little bit of dew. So you measure the surface of the ground underneath one of the trees: 33 degrees F. So it appears that the tree and the ground beneath it exchanged some radiant energy during the night, which slowed down the cooling of the ground in that location — thereby explaining the pattern of the frost.
Another mystery you might have experienced is when you walk by a hot or cold vertical surface. If you walk by a west-facing brick or rock wall just after the sun sets, then you may distinctly feel the heat radiating from that wall. The wall was warmed by the sun all afternoon, and now, even at dusk, the wall is still much warmer than anything else around you. Consequently the radiant energy coming from the wall is stronger than the energy coming from any other direction. Now imagine that you walk by a glass window pane inside your house early on a cold winter morning, and it seems that you can feel cold radiating from the glass. But cold is simply an absence of heat, and therefore it cannot radiate. So what is going on?
In this case, as you stand near the cold pane of glass, your skin is radiating energy towards the window, and the glass is radiating a much smaller quantity of energy back at you. The glass pane is by far the coldest object in the room, and therefore the skin that faces the glass is losing energy at a faster rate than the rest of your skin, causing the temperature to drop faster on that one side of your body. For some people this is a detectable phenomenon, and it feels like the opposite of standing in front of a warm brick wall.
High Temperatures and Thermal Radiation
Up to this point we have been talking about the kinds of temperatures that are normal for humans to experience. Within this range of temperatures, the energy given off as thermal radiation falls within the infrared range. However, the peak wavelength of the emitted infrared light varies according to the temperature. And that is the principle upon which infrared cameras and other thermal imaging devices work. These devices are tuned to detect radiation in the infrared range, at a wavelength that warm bodies emit, but that cooler objects do not. In a thermal image, humans and other warm objects stand out sharply from the cooler objects around them. If you did not emit radiation, then you would not be detectable by such a device.
As long as we restrict our discussion to temperatures that are normal for humans to experience, then it makes sense to equate thermal radiation with infrared radiation. And because all of the objects around us exchange heat energy by means of infrared radiation, then it is easy to assume that infrared radiation is itself a type of heat, often called “radiant heat”. But in fact, infrared is a type of light, not a type of heat — because heat is all about the vibratory motion of molecules of matter, and light contains no matter. When object A emits thermal radiation, then some of its heat energy is converted into light energy. When object B absorbs that thermal radiation, then the light energy is converted back into heat energy.
So what happens when temperatures exceed what a human being would normally experience? If you heat an object to several hundred degrees above room temperature, then its thermal radiation shifts to shorter wavelengths. If the object gets hot enough, then a tiny portion of this energy will be emitted as visible light. You see this every time that the coils on an electric range glow red hot — or the coils in a toaster or toaster oven. When an object is just barely hot enough to glow, then it will give off a faint orange-red color. Most of the emitted energy is still in the infrared range, but a tiny bit of it is in the red end of the visible light spectrum.
In most households, there are few objects that get much hotter than the red-hot coils mentioned above. (If the red glow is clearly visible, then the temperature of the coil is probably in excess of 1000 degrees F. / 540 degrees C.) But if you have any incandescent bulbs in your house, then here is a surprise: The temperature of a typical incandescent filament, when the bulb is lit up, is around 4600 degrees F. / 2550 degrees C. This is hot enough to have a reasonably good distribution of light energy across the entire spectrum of visible light, although still somewhat weighted towards the red end. However, 90% of the light emitted is still in the infrared, which is why incandescent bulbs waste so much energy.
But what if you heated an object to a temperature higher than the filament of an incandescent light bulb? What would happen? For the best answer, we simply need to look to the stars. There are red stars, white or yellow stars, and blue stars. The colors of stars are determined by the temperature. Red stars are relatively cool — not even as hot as an incandescent filament. White stars are hotter than an incandescent filament, with a good balance of wavelengths across the visible spectrum. Blue stars are so hot that the bulk of the emitted light occurs at the blue end of the visible spectrum. So while we have a cultural association of red as a “warm color” and blue as a “cool color”, in terms of the stars (and anything else that is hot enough to emit visible light) it is actually the other way around.
Summary
To sum up, you are now familiar with four key ideas about thermal radiation:
1) All objects give off thermal radiation, except those at a temperature of absolute zero (-460 degrees F. / -273 degrees C.).
2) The amount of thermal radiation emitted by an object is determined by the temperature of the object, increasing rapidly as the temperature increases.
3) Thermal radiation is emitted across a band of wavelengths determined by the temperature of the object, gradually changing from infrared light to visible light as the temperature goes above approximately 1000 degrees F. (540 degrees C.).
4) Objects that share an environment will typically exchange heat energy by several mechanisms. Thermal radiation is one of those mechanisms, often a very important one.
With these ideas in mind, you can now solve mysteries that made little sense before, such as why frost can form on the ground when the thermometer says 35 degrees F., or why a blue star is hotter than a red star. Armed with this new knowledge, what other little mysteries can you solve?
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