This is a guest post composed as part of the NSF Science: Becoming the Messenger workshop, Lawrence KS January 27, 2011
When I was checking out at the local grocery store, I noticed that I could buy a laser pointer at register along with packs of gum and candy. Lasers can be found nearly everywhere, ubiquitous in our culture (thanks George Lucas); however most people do not understand the fundamental difference between light from a laser and that from the sun or a light emitting diode (LED). In fact, most people do not realize that laser light is NOT produced from natural sources (except for some distance astronomical sources) and is a completely human invention on the earth. What makes laser light different than that from the sun? The light from a laser is coherent, which means that the light produced by the laser is in phase and monochromatic, the peaks of all waves are aligned like an individual musicians marching together in a drum corps playing the same tune. The sun produces incoherent light, where the waves are all out phase with each other and at different frequencies, like each musician marching to a different drummer and each playing a different tune. Ever see a laser spot ‘speckle’ on a wall? Wow, you are directly seeing the coherence with your eye. You will never see sunlight speckle like that. If sunlight is natural light, a laser is very unnatural light.
http://thescienceofacne.wordpress.com/category/light-and-laser-therapy/ - Brian Washburn, Kansas State University