Q1.
Place the black sheet of paper undisturbed near an open window (Fig. 8.6a), or in the garden, for a few hours. What do you observe?
Answer
Tiny particles have settled on the surface of the sheet. Under a magnifying glass (Fig. 8.6b) you can see that they are of different sizes, shapes and colours — fine grey dust, specks of soot, bits of pollen, a stray fibre of cloth.
| Sheet | Observation |
|---|---|
| Fig. 8.6a — at the start | Black, clean, free of any visible dust particle |
| Fig. 8.6b — after a few hours | Small particles settled all over the surface |
Why it happens: These particles were floating in the air all along. Once the sheet lies undisturbed, the particles slowly settle on it under gravity and stay there. This shows that dust particles are suspended in the air. A black sheet is chosen deliberately — light-coloured dust shows up clearly against a dark background.
Check it yourself: Repeat the activity with one sheet near a busy road and one inside a closed room. The difference in the amount of dust tells you why the number of dust particles varies from place to place.