Science Questions

Why does laser light appear granular?

Sun, 19th Feb 2012

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Question

Bob asked:

Why does laser light appear to be granular when you actually see it propagating through the air?

Answer

Philip -  I think what heís asking about is a phenomenon known as laser speckle which seems to occur on most lasers when they're propagating through space.  If you shine a laser beam onto a wall, you'll see this speckling effect.  Itís a time varying phenomena due to intrinsic noise in the laser beam and itís interfering with itself, and itís a property that's unique to coherent sources of light so it only occurs with lasers.

Chris -   Mike mentioned that lasers shining on skin produce a speckly pattern.  Is that similar?

Philip -   Yes.  The surface of your skin, at a microscopic level, is quite rough.  Light which strikes this surface will reflect and scatter in all sorts of directions.  All surfaces will have a degree of speckle but skin especially so.

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