My job sometimes puts me in places with radiation hazard signs. Of course, being a human on planet Earth means that we are constantly exposed to a variety of radiation off all different types. Scientist categorize many types of radiation, most of which are not harmful.
Radiation can be defined as the transmission of energy from one point to another via particles or waves. When most people hear the word radiation they think of the Big Bad: hazardous radiation. Typically, these types of radiation come from nuclear reactions, and too much exposure to this kind of radiation can lead to things like cancer or worse (yeah, there’s worse). We are exposed to small amounts of nuclear radiation all the time; after all, it is naturally occurring. There are four common types of nuclear radiation, called alpha, beta, gamma, and neutron radiation. Neutron radiation is composed of – you guessed it – neutrons, which are the electrically uncharged particles in the cores of atoms. Under the right conditions, neutrons can be ejected from the core (or nucleus) of an atom at incredible speeds, and because neutrons are heavy (in relation to atoms) they act like bullets if they strike another atom. Since we are all made of atoms, neutron radiation can really screw with us. Neutron radiation got its name because when it was discovered scientists knew right away what it was. Scientists are not always so lucky, which is why the other nuclear radiation types are named alpha, beta, and gamma, despite the fact that they are very different from one another.
Back at the dawn of the nuclear age, around 1900, Ernest Rutherford studied radiation that came from the natural decay of radioactive materials, something we talked a bit about in a previous article. You may have heard his name from the “Rutherford model of the atom,” which was the first theory of atomic structure that was on the path towards correct. The Rutherford model of the atom – though not entirely accurate – is taught to school children because it is easy to grasp and was correct enough to give us atomic power! So when I say that it is not entire accurate you can see that, in science, being close to right can get someone pretty far. Rutherford theorized that atoms have massive nuclei that possess a positive electrical charge, and that an atom’s nucleus is orbited by considerably less massive electrons, which carry negative electric charge. Critically, the Rutherford model says that the atom is mostly empty space. All of these conclusions came from Rutherford’s experiments with radiation, because if the structure of the atom is not this way then it is impossible to explain the results of his experiments. (If you’re wondering where the Rutherford model is wrong, it has to do with the orbits of the electrons, which are far more complicated than anyone ever expected. In order to understand electron orbits, a number of breakthroughs from quantum mechanics were needed. Rutherford’s basic theory about a massive nucleus, the locations of positive and negative electric charges, and the idea that the atom is mostly empty space are all accurate.)
Rutherford was experimenting with how much aluminum foil was needed to stop nuclear radiation, because that’s the kind of thing that constituted a science experiment in 1900. He discovered that some radiation could be stopped by a sheet of aluminum foil as thin as 0.002 centimeters. The rest of the radiation could be stopped with a sheet of aluminum about 100 times thicker. Rutherford concluded that there must be two different types of radiation, so he called the first alpha and the second beta, after the first two letters of the Greek alphabet, because physicists love the Greek alphabet. It was determined that beta rays carried a negative electric charge, and were in fact electrons. Beta radiation is the electron equivalent of neutron radiation – fast moving electrons capable of disturbing the structure of atoms. Alpha radiation was discovered to actually be the nucleus of helium atoms, carrying a positive electric charge because its two electrons have been stripped off. Exactly why alpha radiation is made of helium nuclei is beyond what we’re talking about today, so I’ll just say that there are certain processes in radioactive decay that make it likely that a helium nucleus will be spit out. Because the helium nucleus is no much more massive than the electron, it travels at lower speed and is more likely to bump into another atom (because it’s physically larger as well), so it can’t penetrate as much aluminum. In fact, alpha radiation can be stopped by your skin, making it a low hazard radiation as long as you’re not exposed to a significant amount of it.
Rutherford originally missed the third type of radiation, which he later named gamma after the third letter of the Greek alphabet. Gamma radiation can penetrate an incredible amount of matter. Scientists realized that gamma radiation was very similar to the previously discovered X-rays; both were types of electromagnetic radiation. However, gamma radiation has much higher energy than X-rays. Gamma radiation has so much energy that it can knock electrons off atoms, turning the atoms into positively charged ions. If this happens in your body, then those ions can interact with other atoms in your body, causing all kinds of problems. Gamma radiation is termed “ionizing radiation” for this reason, and ionizing radiation is often very hazardous.
But what is electromagnetic radiation? If that’s what gamma rays are then electromagnetic radiation sounds dangerous.
In the 1860’s, James Clerk Maxwell developed a set of math equations that explained all existing experiments concerning electricity and magnetism using a coherent set of ideas. These equations beautifully show that electricity and magnetism are connected and are actually two aspects of the same physical phenomenon. There is not “electric” and “magnetic” physics, there is “electromagnetic” physics. This understanding was a breakthrough, transforming a significant part of our understanding of the Universe. One of the many things that Maxwell showed with his equations was the physical mechanism of light. It was known that light was a wave, but every wave must travel through some medium. Sound waves travel through air (or other matter); without air there can be no sound, which is why in space nobody can hear you scream. Maxwell showed that light waves travel as electromagnetic fields, and as a result light is a form of electromagnetic radiation.
Waves can be characterized by their frequency, which is how much time is required for a wave to repeat itself. Alternatively, a wave can be defined by its wavelength, which is the distance it takes for a wave to repeat itself, but wavelength and frequency are related by the speed that the wave travels, so they are not defined independently of one another. When Maxwell showed that light was an electromagnetic wave he also demonstrated that there was no limit as to what frequency electromagnetic waves could possess. Until then, scientists thought that visible light was all that existed. Maxwell showed that the rainbow (red, orange, yellow, green, blue, indigo, and violet) was only a small part of the electromagnetic spectrum. Scientists quickly began exploring what was beyond violet and below red.
The amount of energy in an electromagnetic wave is directly related to its frequency; the higher the frequency the higher the energy. Red light has a lower frequency than violet light, so the area of the electromagnetic spectrum immediately “below” red was named infrared, and the area “above” violet was termed ultraviolet. Infrared light was discovered to efficiently convey heat, which answered an ancient question about how heat is transmitted from the Sun, because it was found that the Sun emits a lot of infrared radiation. Radio waves are another type of electromagnetic radiation below even the infrared, and our mastery of this type of radiation gave us FM radio, satellite communications, cell phones, WiFi, and a plethora of other communications technology which has transformed our society in the last 150 years. Ultraviolet light is given off by electric sparks, and though it lacks the energy to be ionizing radiation, it does possess enough energy to induce some types of chemical reactions. Ultraviolet waves from the Sun help our bodies form vitamin D, but absorbing too much ultraviolet light causes sunburn, making ultraviolet radiation an excellent example of how radiation can be both beneficial and harmful.
Beyond the ultraviolet lies the even higher energy radiation named X-rays, so called because when they were discovered nobody had any idea what it was so it was given the placeholder name “X.” Here’s a science pro-tip from history: don’t ever give anything a placeholder name because no two people are ever going to agree on a name change. Everyone knows that X-rays let doctors see through flesh to reveal bones. This is possible because bones are made mostly of calcium, which likes to absorb X-rays of a certain frequency. X-ray film records a lack of X-rays everywhere there is bone, and lots of X-rays where there isn’t bone.
Keep in mind that humans like to categorize things into nice little boxes. The problem is that when we’re talking about something that varies continuously like a wavelength (or frequency) it is meaningless to draw a line and say “below this line is A, and above it is B.” If you don’t see what I mean just picture a rainbow in the sky. Can you point at the exact spot that divides the color red from the color orange? The spot you pick will almost certainly be different from the spot I pick. A spot (or more accurately, a frequency) can be defined, but we all have to agree on exactly what that is. When we are exploring the parts of the electromagnetic spectrum outside visible light this is even more difficult. Where do X-rays end and gamma rays begin? We can’t see either of them. We can only define them by how they interact with matter. As a result, scientists can’t agree on where the line is between X-rays and gamma rays. The good news is that the distinction itself is useless. We can perfectly characterize electromagnetic radiation by its frequency (or wavelength). We can say “this is electromagnetic radiation with a wavelength of 10 picometers (0.000000001 centimeters)” and there is no confusion among scientists about how it behaves. It would be perfectly accurate to define all electromagnetic radiation beyond the color violet as being ultraviolet radiation. X-rays could then be referred to as “ultraviolet light with a wavelength between 0.001 and 10 nanometers,” with one nanometer being 0.0000001 centimeter. So why don’t we do this?
The reason is history. When X-rays were discovered we didn’t know what they were. We didn’t even know if they were waves (like light) or particles (like electrons). Our discovery of X-rays came well ahead of our understanding of the nature of light. When it was found that X-rays were just a type of electromagnetic radiation there was already a large body of work concerning “X-rays.” To rename them would have just been confusing. As we discussed above, gamma rays were given their name simply because they were the third kind of radiation found to come from an atom. Scientists gave the light just above visible violet the commonsense name ultraviolet. The first X-rays detected had a frequency above what we called ultraviolet, so X-rays were given a range of frequencies, and a nice labeled box was made. Gamma rays were found to have frequencies even higher than X-rays, and since we already called those gamma rays, scientists defined X-rays as having frequencies lower than gamma rays. Historically, gamma rays are the highest energy electromagnetic radiation, so gamma rays are everything with higher energy than X-rays.
This is all just word play. When you boil it down there are two types of radiation: particle (being made up of tiny bits of matter) and waves (being electromagnetic radiation). Electromagnetic radiation is easily defined by its frequency, which represents how much energy the radiation carries. The types of particle radiation are far more varied, and depend on what the particles are (electrons, neutrons, helium nuclei, etc) and how fast they are moving (speed is part of what determines energy, which helps explain why a bullet can kill you but a BB just hurts). Once we understand how generic the term radiation is, it becomes easy for us to accept that radiation is not the Big Bad that most people think it is. Life couldn’t exist without radiation. Some radiation is hazardous, and some is not, the same way some food is good for you and some isn’t. So let’s not judge radiation based on a few bad particles and waves. Let’s accept it for all the good it does for us, and tolerate its few negative aspects.