a. General. Lasers have three important qualities which make laser light very
special: intensity (brightness), directionality, and coherence.
(1) Intensity (Brightness). An ordinary light bulb generates and releases
photons when the filament is excited (the bulb is turned on). A laser, on the other hand,
stores the photons, building up pressurized light energy. When the photons are finally
released there are so many more than a light bulb could possibly release in the same
amount of time, that the resulting beam of light is very bright and very intense. A laser
beam has greater intensity than any other light source.
(2) Directionality. A laser can be thought of as a kind of light gun which
aims photons along the laser rod and fires them out one end like bullets. The tightly
packed photons all go the same way as they leave the laser rod; the photons do not
scatter or spread out the way ordinary light does. Regardless how far the laser light
travels, it keeps going in exactly the same direction spreading very little; a laser beam is
said to be highly directional.
Figure 2-21. Intensity.
(3) Coherence. Like a unit of soldiers marching in review, the photons in
laser light are all lined up together, evenly spaced, and exactly in step. Think of the
members of an Army unit marching in review. They come on the field, each soldier
marching in the same direction and at the same speed. See figure 2-22. Photons in
laser light move the same way--all lined up together, evenly spaced, and exactly in step.
In other words, the photons are well-organized, meaning coherent. See figure 2-23.