Why does laser light appear granular?

19 February 2012

Question

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

Answer

Philip - I think what he's 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. It's a time varying phenomena due to intrinsic noise in the laser beam and it's interfering with itself, and it's 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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