Why does laser light

March 20, 2005 8:40 AM

Why does laser light "sparkle"?

I noticed the laser light sparkles, it's really apparent if you shine it at something that difuses the light, like some semi clear plastic, or cloudy water. The light has a quality to it i've never seen anywhere else. It sparkles. I'm just wondering if anyone knows why?

posted by joelf to science & nature (10 comments total)

I'm doing this just by applying what seems logical to me. An expert can correct me but I'd say it's because mainly because it is a concentrated beam of light, but also perhaps because the beam only covers a narrow spectrum of light (colour).

It's not the laser that's sparkling, but the stuff (dust, particulate) that's in the path of the laser that causes the laser to sparkle by a) blocking light in the path b) constantly moving through the path

You see the same thing happen with dust when bright sun shines through a window, so it's not a unique property of a laser. However, since a laser is so narrow and covers only a small spectrum of light, and not nearly as diffuse, I think the phenomenon is more pronounced.

Stopped physics after junior college and went to a lot of science centers as a child. And like somone else on ask.metafilter said "once tried to scan a hand drawn upc code"
posted by furtive at 9:18 AM on March 20, 2005

Yes. It is known as laser speckle. Laser light different from ordinary light from, say, a lightbulb in two ways: it is mono-chromatic (one color, to a very very good approximation) and coherent. This last term means that all the peaks and troughs of the light wave line up over very long distances. When laser light scatters off of surfaces, these peaks and troughs can interfere to produce bright and dark spots. If the surfrace is regular, one sees a diffraction pattern. If the surface is random and diffuse, one sees laser speckle. Try this interactive guide.
posted by fatllama at 9:20 AM on March 20, 2005

I think what you are seeing is "speckle". Laser light is spatially coherent. The phase of the light is pretty constant across the beam. Where the peaks of intensity line up it is very bright. In other places, the phases cancel and it is dark.

Speckle is particularly noticeable on diffuse surfaces where the surface roughness causes the path length to vary on the order of a wavelength. This adds a significant spatial variation in brightness. Then, slow time variations in the source or the path causes a twinkling effect.

http://repairfaq.ece.drexel.edu/sam/laserioi.htm#ioiscs
posted by Instrumental at 9:22 AM on March 20, 2005

Well I'll be. :-)
posted by furtive at 9:34 AM on March 20, 2005

You can duplicate the interference effect by superimposing two sheets of thin, open weave fabric or window screen.

The monochromatic nature of the light is what makes it seem 'brighter' than it really is. Photocopy a piece of magenta flagging tape or 'fluorescent' paper against a piece of white paper, and you'll find that it's darker than white, yet it seems lighter. Imagine how a flute note would hurt your ear when amplified to only half the volume of the whole orchestra.
posted by weapons-grade pandemonium at 9:59 AM on March 20, 2005

As others have indicated, laser speckle is caused by interference patterns in coherent light when reflected from a surface like a room wall. Laser speckle has the interesting property of forming an extremely sensitive eye test.

The interference patterns form on your retina no matter how your eye is focused for distance. On the other hand, the wall or background from which the light is reflected will be in focus only for only one distance. Depending on whether you are far or near sighted, the image of the background will be focused slightly in front or behind the retina, respectively, while the speckles will appear focused precisely on the retina.

If you move your head from side to side, the speckles will appear to move opposite the apparent motion of the wall if your are near sighted and in the same direction as the apparent motion of the wall if you are far sighted. The worse your vision, the faster the apparent motion. If your vision is perfect, the speckles will not appear to move. This is a parallax effect caused by the background being focused in front of or behind the speckle image.

If you have corrective lenses, you may see opposite effects with and without your lenses. The test is so sensitive that even if you have "perfect" vision, you may see differences throughout the day as the inter-ocular pressure in the eye changes its shape slightly.
posted by JackFlash at 12:48 PM on March 20, 2005

That's very interesting, JackFlash. I noticed this effect at Christmas with the LED tree lights. Nifty to know why these things happen.
posted by weapons-grade pandemonium at 2:25 PM on March 20, 2005

(anyone else find that the blue LED lights makes them feel nauseous? dunno what it is about that colour, but it just makes me ill.)
posted by five fresh fish at 5:42 PM on March 20, 2005

Very cool, JackFlash. I've also read that a robust source of random numbers can be dervied from laser speckle off of a diffuse surface.
posted by fatllama at 8:59 PM on March 20, 2005

Cool JackFlash, I'll have to try that out.
posted by bugbread at 7:45 AM on March 21, 2005