Sparkles and Twinkles…

Have you ever looked at the spot created by a laser pointer and wondered why it looks all sparkly? The fuzzy looking light pattern created by laser beams isn’t an optical illusion, it’s an actual phenomenon. It goes by the name of speckle, and it’s really quite interesting…

Firstly, consider this. No surface which you think is smooth is truly smooth. Not at small enough scales, anyway. Now, your typical green laser pointer has a wavelength of 532 nanometres, and being as a laser is just light, it will scatter off any surface it strikes. So if a green laser pointer is shone at any surface which has a “bumpiness” larger than 532 nanometres, all of those waves of light will suddenly stop being cleanly in phase with each other as they were in the original laser beam, and start being out of phase. Because of how they reflect off the surface imperfections in the material, they interfere with each other randomly, superposing and giving bright spots and dark spots in a speckled pattern, hence “speckle”. It’s exactly like acoustics and how in an empty room, some sounds seem louder or quieter depending on where you might be standing in the room. Just on a much much smaller scale.

Weirdly, all light does this. It’s just that regular white light has so many different wavelengths in it, that it averages out so you see no speckles. Given the right conditions though, you can. Squinting, you may sometimes see speckles from your eyelashes, and I know that in the right light, you can see speckles from fingernails.*

Speckle actually has an application in astronomy, as a way of cleaning up images. All light that passes through Earth’s atmosphere is subjected to speckle as it scatters of all of the molecules that make up the air. And air is constantly moving (which is why stars “twinkle”), so it causes a single point of light from a star to blur and appear smudgy in a long exposure image (the amount of detail you can pick out with all of this going on is normally referred to as ‘astronomical seeing’). This nice little animation should give you some idea of what I mean. Speckle imaging takes a series (sometimes quite a big series) of extremely short exposures instead and uses them to cancel out the effect of the atmosphere. The end result is that you have a much better image of the point of light coming from the star, without all of the atmospheric mess you’d normally have to deal with!

In short, lasers are not only pretty but they’re full of interesting science.

That is all.

* Unfortunately, I’m not sure exactly what those conditions are. I suspect they’d need the right kind of light source though. Evening sunlight would probably be best, because it’s strong, highly directional and partially filtered by the atmosphere to remove the bluer frequencies. Just please don’t go staring at the Sun, and if you feel you don’t care about your eyesight and want to anyway, don’t say that some guy on the internet told you to, ok?

Image credits:
Laser Speckle – flight404, flickr
Speckle Imaging – Wikimedia Commons

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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