MadSci Network: Physics

Re: Why is house dust always grey...???

Date: Wed Oct 6 01:13:40 1999
Posted By: Matthew B. Weyerich, Technical Coordinator,ES&R Dept., CPI Corp.
Area of science: Physics
ID: 934915173.Ph


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 

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 

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 

You may see things otherwise. If so, please feel free to e-mail me at 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,


Just Dust: http://encarta.

Mie Scattering: http://covis.

A cool page about color and light: http://www.cs

Dust Mites! (Careful. The first picture is scary!):

Outer Space dust with little green "mites": http://antwrp.gsfc.nasa.go

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