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While it remains true that light exhibits the properties of an electromagnetic wave as described on the page on Light as a Wave, there are other characteristics of light, discovered more recently, which imply that light also partakes of some of the properties of a physical manifestation. In this context, light behaves in some ways as if it consists of discrete particles rather than infinitely variable waves. These apparent particles have been designated photons.

Some of these characteristics are:

• Any single photon has a fixed, discrete energy level. 
• Each color of light has its own unique energy level. It is not possible to increase or decrease the energy of that single photon without changing its wavelength, or else absorbing it completely and thereby ending its existence. 
• The intensity of visible light can be increased or decreased only by changing the number of photons present. 
• The same rules hold true for electromagnetic phenomena outside the visible range. 

Actually, photons are not particles in the physical sense that we normally associate with that word. Rather, they consist of discrete bundles of energy which are fixed in magnitude. As a result, each photon takes on some of the characteristics of a physical particle.

Viewed in this context, light still does not change its basic behavior. These apparent particles are electrically neutral, so they tend to travel in straight lines, without being affected by either magnetic fields or electrical fields.

If photons were actual physical particles, we would have trouble using them to explain some of the observed behaviors of light. For example, when light passes from a vacuum to a denser medium, such as Earth's atmosphere, it slows down in accordance with the density of the medium. This much, at least, makes intuitive sense. However, light then maintains a constant speed through the new medium — it does not continue to slow down as it continues to move. This does not seem to make much sense for physical particles, which should be subject to friction effects in a non-vacuum. Furthermore, when the light leaves the denser medium for a less dense one, it speeds up again. Definitely not the behavior one would expect from any kind of particle.

But if we examine a photon as a bundle of energy that simply exhibits some of the characteristics of a physical particle, things begin to make more sense. We know by experiment that a photon can transfer its energy to an electron. The photoelectric effect occurs when photons of sufficient energy actually kick electrons off of the surface being struck by light. But even if a given electron hasn't received enough energy from a photon to free it from its material surface, it can receive enough energy to raise it to a higher orbit around its parent nucleus, or even free it from that nucleus. In such cases, the electron can hold that energy for a period of time before falling back to its usual lower-energy orbit and releasing the energy again. This effect explains many phenomena that we can observe directly.

When the photon impacts with the electron, it imparts its energy to the electron. There are several possible results, depending on the energy in the photon:

1. If the photon has insufficient energy to boost the electron to its next higher possible orbit, the electron cannot hold the energy, and releases it again at once, as a photon that matches the incoming photon. The direction of the released photon depends on the nature of the material substance and the energy of the photon itself, so we get phenomena such as reflection and refraction. 
2. If the photon has exactly the energy needed to boost the electron to the next higher allowable orbit, the photon will disappear as all of its energy is imparted to the electron. This is a quasi-stable situation; either this electron or another orbiting electron will seek to lose energy by dropping into the vacated orbit, and will release a photon of exactly that energy when it does so. 
3. If the photon has eneough energy to boost the electron beyond the next orbital energy level, and possibly to a yet higher orbit around its nucleus, it will do so, and the electron will emit a lower-energy photon if necessary, as it initially drops to the highest-energy orbit it can reach. In the meantime, however, another orbiting electron will lose energy by dropping into the vacated orbit, and will emit a photon of its own as it does so. We see this phenomenon in fluorescent lights. Here, the actual source of light energy is UV light produced by a mercury vapor arc through the glass tube. This would normally be very damaging to the eyes, were it not for the phosphors coating the inside of the glass. That coating absorbs the UV light and emits visible light in return. 
4. The photon doesn't always give up all of its energy to the electron it strikes. Under some circumstances, it only gives up part of its energy to the electron, and both a higher-energy electron and a lower-energy photon leave the point of impact. This is known as the Compton Effect. A practical example of this is found in greenhouses, where some wavelengths of incoming sunlight are converted to longer-wavelength infrared (heat) photons, which are then primarily reflected by the glass panes and are therefore trapped inside the greenhouse. 
5. Some substances absorb the energy of most incident photons and either transmit (eg., a colored filter) or reflect (eg., a painted surface) photons of a specific amount of energy only. The chlorophyll in green plants gets its energy by reflecting only green light, and absorbing the energy of photons of other colors. 

So what is a photon, in a scientific sense? Let's take a look at that on the next page: Characteristics of a Photon.

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