Bacteria-like projections when shining a laser through a magnifying glass.?

          I was recently shining my new green laser pointer through a magnifying glass, and the beam went through the little circle near the bottom of the lens that magnifies things more than the rest of the lens. The laser dot got very much larger, and there were odd little dark circles surrounded by larger, transparent circles. I also noticed the same thing, but with longer, oval-like projections instead of circles. What are these projections of? I'm guessing dirt, hair, bacteria, etc. Can anyone tell me? I want to know because it's a pretty cool effect and it would be cool to know if I'm looking at bacteria or something like that.

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orksecurity5 years ago
Another possibility: If you look at an area lit by a laser beam, you will notice that there's an odd granular texturing to the light. My understanding is that this is an interferance phenomenon.
ALogan97 (author)  orksecurity5 years ago
It's not that, it looks more like this: http://www.flickr.com/photos/raytracing/4942214591/
But they don't move, at least not that much
That is really, awesome! It would have saved a lot of speculation if you had uploaded a photo of what you're seeing; I could have answered it right away.

What you are seeing are circular diffraction patterns caused by the laser light bending around microscopic objects smaller than your system can resolve visually. In telescopes, these are called "Airy disks" (look it up in Wikipedia).

Each speck of dust (or microbe in the Flickr video) behaves like a point-source of waves. Those waves interfere with one another, giving you alternating bright and dark rings on the projection.

The diameter and spacing are determined by the wavelength of the light (532 nm for a green laser), the diameter of the pointer source (aperture) and distance to the projection screen.

What is most personally amusing for me is that I just finished dealing with some clueless idiot on a completely different forum, who neither understood nor believed that Airy disks existed (he thought he was able to photograph the actual disks of distant stars with his 14" telescope!).
ALogan97 (author)  kelseymh5 years ago
Could you repost this as an answer instead of a reply so I can mark it as best answer?
ALogan97 (author)  kelseymh5 years ago
I was going to add a picture, but it unfortunately turned out to be pretty difficult to operate a magnifying glass, laser pointer, and a camera all at once.

I think I understand what you're saying. So the light refracts off of dust/microbes and spreads out, displaying a silhouette of whatever it refracted off of, with the size varying according to the aperture and wavelength of the beam and the distance to the projection screen? That makes sense.

And really? I don't really understand astronomy that much, but I'm assuming with a telescope that size it wouldn't be possible to be able to see that much detail of a star unless it was the sun or something.
I think you've got it. The light diffracts -- that is, the waves bend, like water flowing past a rock -- around the microscopic objects. Those waves, when they reach the screen, interfere with one another (that is, high points and low points add to zero, high+high or low+low add to bright). If you work out the math in detail (look up "diffraction" in a decent high-school physics text), you'll get a central dark spot surrounded by alternating light and dark rings.

Your assumption is exactly right, as can be shown with elemenary optics. Hence my use of adjectives :-)
ALogan97 (author)  kelseymh5 years ago
But if light bends around it, wouldn't that be refraction, not diffraction? Diffraction is when light spreads out after passing through something, while refraction is when light changes direction due to hitting an object at an angle. Or would it be some of both? I somewhat understand the interference, mostly just the math of high and low points canceling out to cause light and dark though, not really the whole concept.

And yes, the adjectives used were quite effective.
Refraction occurs when light is bent at the interface between two media with different indices of refraction, for example between air and water, or vacuum and polished amorphous quartz. Refraction is strictly a phenomenon of transmission -- the light passes through the interface at an oblique angle, and than angle is changed according to the relative indices of refraction on either side.

Diffraction occurs when light hits the edge of an opaque barrier. In Huygen's terminology, the edge becomes a new source of spherical or cylindrical waves, which travel away from the diffracting object. Diffraction may be transmissive (e.g., passing through a slit and making stripes), refractive (e.g, passing around a small object), or reflective (e.g., the rainbow pattern you see on a CD).
ALogan97 (author)  kelseymh5 years ago
Ok, dictionary.com didn't say that, but it makes more sense.
Sorry; I'm a professional physicist, and I too often assume that everyone else knows the same technical definitions I do. I hope that helped, and thanks again for the interesting Question!
ALogan97 (author)  kelseymh5 years ago
I do the same thing with photography/computer science terms. It did help a lot, thanks!
In that case... You may indeed just be projecting flaws in or on the lens.
ALogan97 (author)  orksecurity5 years ago
Probably. But I tried shining it through a drop of water to se what would happen, and it showed the same things, but they were moving around a lot, and there were lots of large clumps and rod-like structures. Is that just bubbles or dirt? If it is, then what could explain the rods?
Yes, it is. The technical term is speckle.
lemonie5 years ago
sshuggi5 years ago
Yea, it's just dust and scratches on your lens.