Why is the Sky Blue? The sky is blue because of a process called Rayleigh scattering. Light arriving from the Sun hits the molecules in the air and is scattered in all directions. The amount of scattering depends on the frequency, that is the color of the light. Blue light, which has a high frequency is scattered much, much more than red light, which has a lower frequency. So the "background" scattered light we see in the sky is blue. This same process also explains the beautiful red colors at sunset. When the Sun is low on the horizon, its light has to pass through a large amount of atmosphere on its way to us. During the trip, blue light is scattered away, but red light, which is less susceptible to scattering, can continue on its direct path to our eyes. The Laser In theory, the idea behind the laser is very simple. Think of a crowd of people doing the wave at a baseball stadium. Here people stand up for a brief moment and wave their hands up and down in unison. The effect is the appearance of a wave of wiggling people traveling from one end of the stadium to another and back. To start the wave, just one or two people stand up. At this point, the wave is little more than a ripple as few people realize it�s even passing. But each time the ripple travels back and forth across the stadium, a few more people stop eating their hot dogs and are stimulated by the excitement to join the crowd. The wave grows as more people join in. Eventually the crowd is a sea of people swaying and jumping in unison. What started out as a single, lonely person waving has, after many repetitions of the wave, turned into a powerful force following in one direction. This is exactly what happens in a laser. In a laser, there is a little ruby rod that holds atoms. Each end of the rod is polished into a mirrored surface. For example, if you start the wave by briefly shining a light on to the ruby. Some of the light travels parallel to the rod and excites a few more neighboring atoms, causing a little pulse of light to travel to the end of the rod where it meets the mirror. The light bounces off the mirror and travels back through the ruby atom "crowd." On its way back the light stimulates other atoms to give up light (radiate), and that energy is added to the wave of light. As the wave of light reaches the end of the ruby, it hits the mirrored end, and the process repeats itself. Eventually after bouncing back and forth and stimulating enough ruby atoms to join in, the light beam has grown intense and the light waves reaching the mirror are powerful enough to burst through. A beam shoots out of the end, and now you have a laser. Because the light in the crowd stimulated identical atoms to join in, the emerging laser light is of one color and very intense.