Re: Why can people see dust in a beam of light?

We're all used to seeing dust caught in a sun-beam, or the ray of light from a projector.  It's such a 
striking sight that many people assume the light must somehow be attracting the dust.  
Unfortunately, that's not the case; the dust you see as the sun-beam shines across your bedroom 
really is in the air all around you, and you are breathing it in all the time.  The good news is that 
your lungs are fairly good at dealing with a bit of dust.

Interestingly, astronomers also see dust in space.  In fact the particles of space-dust are generally 
much smaller than that which gathers on top of your wardrobe.  The density is also much, much 
less than in our homes.  However, there are situations where we see a light beams reflected off the 
dust and the light is scattered in much the same way, and this is often part of the visual effect you 
see in the images of nebulae taken by the Hubble Space Telescope.