[bob]

bob  asked the Naked Scientists:
   
Hi

Why does the light from a laser appear to be granular instead of a solid colour?

What do you think?

[JP]

Hi Bob,

This has to do with the way in which waves interact with each other.  If you have two waves that overlap, the total wave in the region of overlap is the sum of the two waves.  You can imagine waves like those in the ocean--they have peaks and troughs.  If the peaks of two waves line up, the total wave formed by adding them is big (we call this constructive interference).  If a peak lines up with a trough, the total wave from adding them up is small (this is destructive interference).  Those are the basics needed to answer your question.

Light is a wave, so a laser beam can be more or less thought of as a collection of waves coming from every point at the exit to your laser.  As long as these waves go through empty space, they more or less line up so that their interference forms a nice smooth beam of light.  But if some of the waves interact with matter, they start interfering more randomly, and this can lead to tiny spots of constructive or destructive interference in the beam: which shows up as bright or dark spots in the beam.  One cause of this granularity, the technical term for which is speckle, is the light interacting with dust or other particles in the air.

A bigger cause is that usually when you look at laser light, you do so by shining it on a piece of paper.  The paper seems smooth, but on a microscopic level, it's very rough.  This roughness means that the light reflected from it will have a lot of randomness in how it interacts, so you get a pattern that looks, to your eye, to have a lot of speckle.  This is because the roughness of the surface causes the light waves to interact randomly within the beam, forming a bunch of tiny spots of destructive and constructive interference.

[yor_on]

nice question and answer.

[JP]

There's a neat experiment you can do, too, if you wear glasses (or if you don't and borrow someone else's glasses).  Shine the laser onto something so you see speckles.  Then either take off your glasses if you wear them, or put on glasses if you don't so that everything becomes a bit blurry.  The speckles will stay clear while everything else blurs.  This is because they're caused by interference on your retina of the light waves that have traveled different distances, so blurring doesn't cause them to interfere any less.

[RD]

Practical use of laser speckle ... http://www.economist.com/blogs/babbage/2011/07/anti-counterfeiting-measures

[yor_on]

That should go into Kitchen science JP, with a good explanation to why it doesn't 'blur'.
And the application for it was very cool RD

[SeanB]

Tried it and yes, the speckles stay the same in 1 eye but the other does change between lenses on and off. Rotating the laser around it's axis of projection has no effect, but turning my head to different angles moves the speckles.

[JP]

Yes, it makes sense that the speckles move or change shape with vs. without glasses, but they should remain "sharp" even if the background is blurry.  The reason is that the glasses should change the way the waves move, making them interfere and cancel/add up in different spots.

Blurriness is caused when your eyes try to focus on objects far away, but since these spots are formed on the retina, your eye doesn't get a chance to blur them.