|MadSci Network: Physics
Bonamy: I had to search for an answer to this one. In my efforts I learned quite a bit about dust mites, dust allergies, dust removal and abatement, etc., but I couldn't find a thing about why household dust appears gray! So, I put on my thinking cap and came up with the following: House dust is made up of small particles which don't reflect very well. (Dust is not often shiny, right?) The many different things which make up "dust" reflect different wavelengths of light , but the particles are very tiny. They don't individually reflect a lot of color information to your eye. Instead, when you look at "dust" your eyes may get more information about light vs. dark than specific colors. (A dust-like exception to this may be a brilliant cobalt-blue toner powder I once played with. No matter how thinly I spread it, it still reflected "blue" to my eyes. Then again, toner powder particles are probably bigger than many dust particles. [I tried looking at household dust under a microscope. There are all sorts of brightly colored things in dust!] Also, toner ONLY reflects. No light goes through the toner particles, as it often does with dust particles.) In my humble opinion, what you're seeing might be a variant of what one sees when looking at a white cloud against the sky: scattering. Dust is made up of small particles. Those particles are much larger than the wavelength of visible light, so a lot of them together can cause what's known as Mie scattering. (Basically, this involves the random scattering of light by many particles.) Mie scattering is why water-vapor clouds appear white. It may be why dust particles appear white-ish, or, "gray." If we consider a HUGE pile of dust, we only see the (multi-faceted) surface reflections. These reflections aren't very directional, organized, or intense. Pretty dull, even though some individual particles of the visible dust in our huge pile might sparkle like the Hope diamond. A thinly distributed coating of dust on a surface might behave the same way. In this case, individual particles would be more distinct, but, you and I would still see "dust" on a "surface". We'd notice the contrast between the dust and the surface. Or maybe, the tiny shadows of the dust particles. If we were dust mites we might think of the dust particles as "boulders" in a "desert". (We'd be a lot closer though, wouldn't we?) I think this may be the key to your question, for, if we could be that small, we might be able to get close enough to witness the pretty colors. Unfortunately, we are both "World Trade Center" sized, compared to dust mites, so we miss some of the details. This happens (I'd bet) because the dust "boulders" bounce the light around a bit and confuse the picture for us. Other than that, I'm not sure. Maybe dust is just made of mostly gray-stuff? (Careful with this! I looked at "work dust" under the microscope, too. It had lots of interesting things in it. From afar, though, it looked distinctly "tan". Different species altogether, I'd bet!;) You may see things otherwise. If so, please feel free to e-mail me at email@example.com and set me straight. (After all, I'm giving you the "dust mite" point of view!) I've really enjoyed thinking about this, and appreciate your fine question! Hope I've helped in some way. Your MadSci, -Matt firstname.lastname@example.org Links: Just Dust: http://encarta. msn.com/index/conciseindex/3A/03A96000.htm Mie Scattering: http://covis. atmos.uiuc.edu/guide/optics/html/mie-scat.html A cool page about color and light: http://www.cs .brown.edu/exploratory/ColorWeb/color_TOC.html Dust Mites! (Careful. The first picture is scary!): http://www.ozemail.com.au/~lblan co/ Outer Space dust with little green "mites": http://antwrp.gsfc.nasa.go v/apod/ap961119.html
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