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The photon is the basic unit, particle, or carrier of light.

The visible light that we see, the x rays that dentists use, and the radio waves that carry music to our radios are all forms of electromagnetic radiation. Other forms include the microwaves which we use to cook food and gamma rays which are produced when radioactive elements disintegrate. Although they seem quite different, all types of electromagnetic radiation behave in similar ways. If you think about it, the shadows of our teeth that are produced by x rays and captured on special film are really not that different from our visible shadows cast by the sun. If x rays and light are essentially the same, why is one visible to our eyes and the other invisible?

We know that visible light comes in many different colors, like those we see in a rainbow. The colors can be understood by thinking of light as a vibration moving through space. Any vibration, or oscillation, repeats itself with a certain rhythm, or frequency. For light, every shade of every color corresponds to a different frequency, and the vibration of blue light, for example, has a higher frequency than that of red light. It turns out that our eyes can only detect electromagnetic radiation for a relatively narrow range of frequencies, and so only those vibrations are "visible." However, other forms of electromagnetic radiation are all around us with frequencies our eyes cannot detect. If our eyes could detect very high frequencies, we could see the x rays which can pass through many solid objects just like visible light passes through tinted glass.

Originally, vibrations of light were thought to be somehow similar to water waves. The energy carried by that kind of vibration is related to the height of the wave, so a brighter source of light would seem to simply produce bigger waves. This idea provided a very effective way of understanding electromagnetic radiation until about 100 years ago. At that time several phenomena were found which could only be explained if light was considered to be made up of extremely small pieces or "wave packets," which still had some of the properties of waves. One of the most important phenomena was the photoelectric effect. It was discovered that when visible light shined on certain metals, electrons were ejected from the material. Those free electrons were called photoelectrons. It was also found that it took a certain minimum amount of energy to release electrons from the metal. The original vibration concept suggested that any color(frequency) of light would do this if a bright enough source (lamp) was used. This was because eventually the waves of light would become large enough to carry enough energy to free some electrons. However, this is not what happened! Instead it was found that, for example, even dim blue light could produce photoelectrons while the brightest red light could not. The original vibration theory of light could not explain this so another idea was needed.

In 1905 Albert Einstein suggested that this effect meant that the vibrations of light came in small pieces or "wave packets." He also explained that each packet contained a predetermined amount (or quantum) of energy which was equal to a constant multiplied by the frequency of the light. This meant that a bright source of a particular color of light just produced more packets than a dim source of the same color did. If the energy, and therefore the frequency, of a packet was large enough, an electron could be freed from the metal. More packets of that frequency would release more electrons. On the other hand when the energy of a packet was too small, it did not matter how many packets struck the metal, no electrons would be freed. This new idea explained all the newly discovered phenomena and also agreed with effects that had been known for hundreds of years. Einstein's wave packets became known as photons, which are somehow like indivisible pieces (like small particles) and also like vibrations. The discovery of this split personality was one of the factors that led to the theory of quantum mechanics.

Light from a lamp consists of photons. Why does the light we see appear to be reaching us continuously instead of in lumps? Well, this is actually easy to understand by performing an experiment with sand. First, we need to fill a plastic bucket with sand and hold it over a bathroom scale. Next, we make a small hole in the bottom of the bucket so that sand will slowly drain out and fall on the scale. As more and more sand collects on the scale, we will see that the weight increases in an apparently continuous manner. However, we know that sand is made up of particles and so the weight on the scale must really be increasing by jumps (whenever a new grain of sand lands on the scale). The trick is that the size of the grains is so small that the individual increments by which the weight changes are too small for us to detect. The same thing happens with light, only in a more exaggerated way. If we look into a lamp (not recommended) there are billions photons reaching our eyes in every second, with each photon carrying only a small amount of energy.



Albert, A.Z. Quantum Mechanics and Experience. Cambridge, MA: Harvard University Press, 1992.

Gregory, B. Inventing Reality: Physics as Language. New York: John Wiley & Sons, 1990.

Han, M.Y. The Probable Universe. Blue Ridge Summit, PA: TAB Books, 1993.

James J. Carroll


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Electromagnetic radiation

—The energy of photons, having properties of both particles and waves. The major wavelength bands are, from short to long: cosmic, ultraviolet, visible or "light," infrared, and radio.


—The amount of radiant energy in the different orbits of an electron around the nucleus of an atom.

Quantum mechanics

—The theory that has been developed from Max Planck's quantum principle to describe the physics of the very small. The quantum principle basically states that energy only comes in certain indivisible amounts designated as quanta. Any physical interaction in which energy is exchanged can only exchange integral numbers of quanta.

Additional topics

Science EncyclopediaScience & Philosophy: Philosophy of Mind - Early Ideas to Planck length