Colloquially, when the term "radiation" is used, it is often just a shorter way of "electromagnetic radiation," even though this isn't the true definition of the word. Consequently, you often see acoustic or gravitational radiation referred to by acoustic or gravitational waves instead.

So for now, let's focus on just the electromagnetic radiation kind.

Electromagnetic radiation is fundamental to the universe and our daily lives, whether it is from the x-ray machines we use to diagnose medical conditions or the light we see every day, including the blue light from our electronics.

You can't see this radiation with your naked eye, but it is everywhere in the universe, even right around you. It is invisible to humans because it is so cold, just over 2.725 Kelvin. This means that this radiation is most visible in the microwave part of the electromagnetic spectrum.

The radiation from the cosmic background radiation in was scattered off the electrons. Thus, photons wandered through the early universe, just as optical light wanders through a dense fog. The universe began 13.8 billion years ago, and the cosmic background radiation dates back to about 400,000 years after the Big Bang. That's because in the early stages of the universe, when it was just 1/100,000,000 its size today, its temperature was 237 million Kelvin, an extreme amount. This meant that atoms present at that time were quickly broken apart into its elementary particles, which are its protons, neutrons, and electrons, turning it into plasma.

High doses of gamma rays can kill cancerous cells in a process called radiation therapy. Lower doses, however, could lead to healthy cells becoming cancerous. The process of radiation therapy kills the DNA of cancerous cells, preventing growth or division with the use of a machine called an accelerator or radioactive sources placed inside the patient. The oncologist's main focus is to target the dose of radiation to the cancer cells as precisely as possible to avoid hurting healthy tissue, which would otherwise cause a whole range of side effects. Harming healthy cells is inevitable, though. Such side effects depend on the area of treatment. They include, when situated at:

These side effects may sound scary, however, the benefits of radiation therapy often outweigh the side effects when does correctly.

Not to mention, gamma rays are also quite effective in sterilizing medical equipment, since they can easily pass through the packaging of medical equipment and kill living tissue such as viruses and bacteria. Obviously, they are not effective for cleaning off debris, though.

Nuclear power is characterized by the very large amount of energy produced from a very small amount of fuel, and the amount of waste produced during this process is also relatively small. However, much of the waste produced is radioactive and therefore must be carefully managed as hazardous material. All parts of the nuclear fuel cycle produce some radioactive waste and the cost of managing and disposing of this is part of the electricity cost (i.e. it is internalized and paid for by the electricity consumers).

All toxic waste needs be dealt with safely – not just radioactive waste – and in countries with nuclear power, radioactive waste comprises a very small proportion of total industrial hazardous waste generated.

Radioactive waste is not unique to the nuclear fuel cycle. Radioactive materials are used extensively in medicine, agriculture, research, manufacturing, non-destructive testing, and minerals exploration. Unlike other hazardous industrial materials the level of hazard of all radioactive waste – its radioactivity – diminishes with time.”

In radioactive decay, three common emissions occur:

As you may notice, these particles were named using the first three letters of the Greek alphabet. This is because scientists were unable to identify them when they were first observed. A while later, these particles were identified. Alpha particles are helium-4 nuclei, beta particles are electrons, and gamma rays are photons.

Alpha Decay

The radioactive decay process that emits alpha particles is called alpha decay. An example of a nucleus that undergoes alpha decay is uranium-238. The alpha decay of uranium-238 is:

238_92 U -> 4_2 He + 234_90 Th

In this nuclear change, the uranium atom "transmutes" into an atom of thorium. In the process, it gave off an alpha particle. Let's look at the symbol for the alpha particle: 

4_2 He

The bottom number in a nuclear symbol is the number of protons. That means that the alpha particle has two protons in it that were lost by the uranium atom. The two protons also have a positive charge of 2. The top number, 4, is the mass number or the total of the protons and neutrons in the particle. Because it has two protons, and a total of four protons and neutrons, alpha particles must also have two neutrons. Alpha particles always have this same composition: two protons and two neutrons.

Beta Decay

Another common radioactive decay process is beta decay, which emits beta particles. A beta particle is simply just a high energy electron that is emitted from the nucleus.

It may not make much sense at first, because nuclei do not contain electrons, yet during beta decay, an electron is emitted from the nucleus. At the same time that the electron is being emitted from the nucleus, a neutron is turning into a proton. You may picture this as a neutron breaking into two pieces with the pieces being a proton and an electron, however, this is not what actually happens.

For convenience, though, we will treat beta decay as a neutron splitting into a proton and an electron. The proton stays in the nucleus, increasing the atomic number of the atom by one. The electron is ejected from the nucleus and is the particle of radiation called beta.

To include an electron into a nuclear equation, an atomic number and a mass number had to be assigned to an electron in order to balance the equation. The mass number assigned to an electron is 0, which is reasonable, since the mass number is the number of protons plus the number of neutrons, and an electron contains neither protons nor neutrons. The atomic number assigned to an electron is -1, because that allows a nuclear equation containing an electron to balance atomic numbers. Therefore, the nuclear symbol representing an electron is:

0_(-1) e

It can also be represented in terms of the letter beta:

0_(-1) β

Thorium-234 is a nucleus that undergoes beta decay. Here is the nuclear equation for this beta decay:

234_90 Th -> 0_(-1) e + 234_91 Pa

Gamma Rays

Gamma ray production is often a side effect of nuclear reactions of all types. In the alpha decay of uranium-238 shown above, two gamma rays of different energies are emitted in addition to the alpha particle.

238_92 U -> 4_2 He + 234_90 Th + 2 0_0 γ

Virtually all of the nuclear reactions, but they are often omitted for simplicity. Nuclear reactions exert much more energy than chemical reactions. Chemical reactions release the difference between the chemical bond energy of the reactants and products. Nuclear reactions release some of the binding energy and may convert tiny amounts of matter into energy. That means that nuclear changes involve almost one million times more energy per atom than chemical changes!