I’ve noticed that the red dot produced by a laser diode (such as that in a laser pointer or barcode scanner) has a distinctly ‘grainy’ appearance - it doesn’t seem to be dependent on the surface on which it is projected; what is happening to cause this? - is it ‘real’ or is it happening inside my eye?
You sure that you see this from a laser pointer? A barcode scanner shines the laser on to a moving mirror, which I suppose causes the effect (I think) you are describing.
What’s “grainy”?
If I may bring in a literary quote:
"Laser light has a particular gritty intensity, a molecular purity reflecting its origins. Your eye notices this, somehow knows that it’s unatural. It stands out anywhere, but especially under a dirty overpass in the middle of the night." – Neal Stephenson, Snow Crash
Gritty purity. Hmm.
Oh. John Wayne.
There is a central red spot, surrounded by a sort of zone that appears to be composed of individual bright spots which become sparser at the edges; it looks a bit like this.
Okay, at this point I’ll have to take a WAG.
The pointer isn’t being held steadily, because it’s in someone’s hand. The light spot moves around.
Either:
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the laser is being pulsed at a sufficiently low frequency for you to perceive individual dots.
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you are viewing the laser with a pulsed background light (e.g. fluorescent lighting) that causes stronger perception of the laser light when the background lighting is dark.
I think it’s a diffraction pattern caused by the size and shape of the aperture, as well as any dust or irregularities in the path of the beam. With an ideal system you should get concentric rings instead of scattered points.
This grittiness is also observable on static holograms, so I don’t think it’s anything to do with hand steadiness. I’d go for scr4’s explanation, minus the dust.
I meant dust on the laser pointer’s lens, and irregularities of the lens. It might still be negligible though.
Another possibility is that there is interference/diffraction is not happening inside the laser pointer, but caused by the irregularities of the target surface. If you point the laser at a white piece of paper, each point on the spot acts as a light emitter and can interfere with nearby points. Maybe. You can find out by trying the pointer on different surfaces noting whether the pattern is cleaner on a smooth shiny surface.
It’s called laser speckle, and, as has been pointed out above, it’s an interference effect caused by the laser light reflected from different illuminated sections “interfering” with each other. It looks strange because laser light is highly coherent, and it’s easier for it to interfere with itself than “natural” light. Nevertheless, you se interference in reflected sunlight, if you know what to look for.
Speckle has an extensive literature (one of my professors made it his specialty), and , far from being a total nuisance, can be used as a tool for measurement.
So just to clarify; this is interference that is happening on the way back from the surface to the eye?
I`ve noticed this too. It appears to be a consistant pattern even when the beam is moved around. So that rules out interference between the eye and the spot. It must be the poor optics in the laser tool itself.
/hijack
I`ve seen a number of laser shows. How do they make a laser beam seem to stop in midair? You may have a beam shoot up into the sky for what seems to be 75 or 100 feet then it just stops. How is this possible?
/hijack ends
Poor optics, indeed! You gotta have good coherence in order to see speckle. The interference is a property of the light waves themselves – interference is when the amplitudes of the electric fields of the light beams “add up” to increase the overall amplitude or to cancel it out (or anything in between). A camera looking at the same spot will see speckle, just as the human eye will.
For you to see the beam at all, there has to be particulate matter scattering the light of the beam back to your eye (google “Tindal Effect” sometime). This is typically smoke, from either a smoke machine or depending on the venue, an eariler fireworks display.
Above the smoke cloud, the scattering lessens dramatically, giving the illusion of the beam “stopping”
I’ve noticed this speckle too, from my laser pointer. Point it at a light colored candle (wax) to really see it diffuse, the speckle seems to be at its max there. It seems correct to me that it’s caused by interference patterns from the microscopic undulations in whatever material you’re pointing it at. cool.
Nitpick: Tyndal or Tyndall effect, otherwise you won’t find anything on Google.
That great reference on oddball physics, Jearl D. Walker’s The Flying Circus of Physics notes this effect in one of its entries (the way a beam seems to abruptly stop). You’re quite right that you can only see such a beam because of scattering, and Walker attributes the abrupt “stopping”: of such a beam to the expoential decay of the beam intensity due to that (and other) scatter. It’s not clear to me why the stopping should be so abrupt, though – scattering should produce a gradual, exponential decay.
What happens if someone in a plane looks down at one of those laser show lasers? Will it harm his retina?
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Exactly, I`ve seen these laser light shows on a clear cloudless night and the beams seem to just STOP. This is clearly intended as some of the effects are created by different lenghts of light beams.
What gives???
Actually, dust particles and water droplets in the air are usually more than enough, esecpailly when the laser is coming ‘towards’ you. Its the same effect ou get when you see the “beams” from spotlights.
You can test this yourself with a hand laser. Point it aiming generally close to you and it’s a lot easier to get an ‘outline’ of the beam from the dust/etc. in the air than it is when the laser is pointing away from you.
Smoke helps, though, to a point.