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I am a student who is interested in scientific facts. I want a simple definition for radiation. Neither too scientific, nor too simple.

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  • $\begingroup$ you do not state what confuses you in the link you gave. $\endgroup$ – anna v Mar 13 '14 at 6:56
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    $\begingroup$ @rishi.. The question is not specific enough: what type of radiation do you mean? $\endgroup$ – Ruslan Mar 13 '14 at 9:20
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    $\begingroup$ @Qmechanic did you really insert the link that changed the level/coherence of the question? You should have given it in a comment. $\endgroup$ – anna v Mar 13 '14 at 11:39
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    $\begingroup$ There are various rationals for including Wikipedia-links in posts. For instance, OP is expected to have done some basic research, such as, e.g., consulted Wikipedia, before asking. In other words, if inserting a Wikipedia link renders the question obsolete, then it is not much of a question to start with; and potential Phys.SE answers (composed by Phys.SE users in a short amount of time) would typically not be able to compete with a Wikipedia answer (perfected by many different Wikipedia authors over the last 10 years). Phys.SE shouldn't try to compete with Wikipedia on its home-turf. $\endgroup$ – Qmechanic Mar 13 '14 at 12:00
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    $\begingroup$ @Qmechanic The OP might be so young that he/she could not know that he/she could find a wikipedia entry. By editing the question you made it seem as if he/she were aware of it and wanting maybe something different. ( hence my first comment) . Possibly the OP is not even aware of the insertion by you, ( except after these comments) You entirely changed the context and partially the content of the question. $\endgroup$ – anna v Mar 13 '14 at 12:29
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Radiation comes from the verb "radiate" and, as the wiki article linked says is used for describing two different physical phenomena. Both have in common that a "source" is radiating energy ( radiation) whose effects appear at a distance from the source.

1) electromagnetic radiation . This includes from radar and radio waves, to visible light, to x-rays and gamma rays. It is emitted from a source according to the electromagnetic equations where there must be a varying electric or magnetic field at the source, with the corresponding frequency of the wave so as to generate it.

In the case of radio and radar waves the source is a metallic rod or system (antenna) where the electric field is varied across it, and the antenna radiates a wave of energy as seen in the answer given by @mhodel above. To see this radiation one needs a receiver which will absorb the electromagnetic wave and decipher the signals it may be carrying.

Visible light appears from incandescent material as the sun, or various lamps. The antennas/sources are the tiny releases of energy of a zillion microscopic electric charges moving in the electric and magnetic field of each other or released by transitions from bound energy states due to the motions of the atoms and molecules at the high temperatures necessary for incandescence. All masses of matter radiate electromagnetic radiation according to their temperature , black body radiation, but most of the radiation is in the invisible part for our eyes: infrared ( we sense it as heat on our skin) and ultra violet ( turns our skin black by reacting with melanin in it) radiation.

X-rays are of even higher frequency than ultraviolet, generated by charges moving in very strong electric fields. They are invisible except when reacting on matter, film, etc, and long exposures are dangerous for living things.

Gamma rays come from even higher energy sources, nuclear sources in transitions of energy states of nuclear matter.

Electromagnetic radiation is invisible in air and when falling on matter it is either absorbed or reflected .

2) massive particles as radiation . When first the radioactive sources were discovered the "action at a distance from the source" seemed to the observers similar to the radiation given off by electromagnetic sources and the description "radiation" was given to describe the "action through air'of the energy emitted by radium and other sources. It was later found out that in addition to gamma rays, energy was also emitted as small charged particles which did not interact with air enough as to be visible but could leave a track in a denser medium and thus reveal their mass and charge. These are described by the answer of @Wayfarer.

Thus radiation is an inclusive attribute of two different types of energy transfers through space, observed in nature and used by us in various situations.

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Disclaimer: I too am just a student, so take this with a grain of salt.

As I understand it, radiation is just another word for electromagnetic waves. Visible light, infrared, ultraviolet, gamma rays, microwaves--they're all radiation. Classically, this is thought of as coupled waves in the strength of the electric and magnetic fields traveling through space in a definite direction, like so.

Quantum mechanically, (this is probably getting too scientific already...oh well) radiation is described in terms of a "photon field" which is modeled as a harmonic oscillator (a mass spring system). The quantum mechanical solution to the harmonic oscillator is that there are evenly spaced energy levels separated by a constant multiple of $$\omega$$ the frequency of the oscillator (in the mass spring example, $\omega=\sqrt{\frac{k}{m}}$, where k is the strength of the spring and m is the mass). We say that the nth energy, $E_n$ is given by $E_n = \frac{h}{2 \pi} \omega n$, where $\frac{h}{2 \pi}$ is just some proportionality constant. The important fact is that the nth energy is proportional to n. In the case of the "photon field" we say that each photon has a certain energy (proportional to its frequency $\omega$) and that the nth energy state is just a state with n photons. In this sense radiation can be thought of as a constant stream of photons.

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  • $\begingroup$ @anna v: Great answer! I would upvote/comment on your post, but unfortunately I don't seem to have enough reputation to do either of those things yet. I'm new to the site and a little confused by these policies. Having a hard time getting started here because most questions are beyond my level of expertise and the few that I do feel qualified to answer nobody seems to upvote. Can't really see how I'll ever get enough reputation to upvote/comment on posts. $\endgroup$ – mhodel Mar 13 '14 at 20:40
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Radiation in this context is radioactive decay. It is when the nucleus of an atom is in an "unstable configuration." Nuclear stability is not perfectly understood, but we have models and hybrid experimental/theory-based formulas. N (number of neutrons) = Z (number of protons) is a good rule of thumb, unless the nucleus is big, where N > Z is preferred. This has to do with energy levels and the charge of the particles, which I won't get in to.

Let's look at three types.

Alpha decay is when the decaying nucleus emits an alpha particle, or a helium-4 nucleus. The resulting nucleus is called a daughter nucleus, and it has lost 2 protons and 2 neutrons. Therefore, it is now a different element.

Beta decay is when the decaying nucleus emits either a positron or an electron, and a third particle, which I won't get into. In this type of decay, one of the neutrons turns into a proton, electron, and a third particle; or one of the protons turns into a neutron, positron, and a third particle. Since it either lost or gained a proton, the element has changed.

In gamma decay, a photon is released from the nucleus. By this process, the nucleus lowers its energy. The element does not change in this process.

If the emitted particle/photon is of high enough energy, it can ionize atoms, hence why it can be dangerous for humans.

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