The presence of dust particles in air can be shown as follows :
We darken a room completely by putting black chart papers on its windows and a black curtain on its door. A small hole is made in the black chart paper fixed on that window, which faces the sun.
Air contains dust particles
A thin beam of sunlight enters the darkened room through the small hole made in the window. We can see the small dust particles moving in the beam of light in the darkroom. Actually, the air in the room always contains some dust particles but they are so small that normally they are not visible to us. These tiny dust particles become visible and can be seen by us only when a thick beam of sunlight falls on them. This experiment shows that air contains dust particles.
