Physics Question #2450

Paul Martos, a 57 year old male from Michigan City, Indiana asks on December 20, 2004,

I have a small laser pointer. The spot produced by the beam is not a solid red circle, but a fuzzy blotch seemingly made of a dozen tiny red spots. This appears on any surface the beam hits. Why does it look like that? I seem to remember reading an explanation that it has to do with quantum mechanics, but can't find it.

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The answer

Barry Shell answered on December 21, 2004

The part that has to do with quantum mechanics is the way the laser light is created in the first place. Many good explanations of this can be found by doing a web search for "how lasers work". The speckles you are seeing are an interference pattern caused by the reflection of the highly coherent, perfectly in phase, laser light as it bounces off an uneven surface, such as a wall. Here is an explanation from Paul Doherty's excellent Scientific Explorations website: "The waves of light from the laser are coherent before they scatter from the rough screen, that is, they are all in phase, the crests of one light wave line up with the crests of all the others. After scattering from the screen the crests no longer line up, the scattered waves interfere constructively to make bright points and destructively to make dark points. We say that the speckles are caused by interference of the laser light scattered from the rough surface of the screen."

Shawn Keet answered on January 12, 2015

If your are seeing the same amount of dots on ANY surface, it isn't likely that the interference pattern is caused by the rough surface onto which the beam is being projected. It is more likely to be caused by imperfections or impurities in the lens of the laser you are using.  After all, quality control on a simple laser pointer is not as high as that on a precision laser instrument, nor does it have to be.  A small defect is enough to cause some of the light to fall out of phase with the rest and create an interference pattern that would be similar on all surfaces and appear larger as the source/surface distance increases.

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