Q1.
Have you ever observed light scattering by particles in your surroundings? Think of some more examples!
Answer
Yes — wherever a beam of light passes through air or a liquid that carries suspended particles, its path becomes visible. That is the Tyndall effect.
- A shaft of sunlight entering a dark room through a small hole or a gap in the curtain, picked out by the dust in the air.
- Sunlight streaming through the gaps between the leaves of a dense tree, or through mist in an early-morning forest.
- The beams of floodlights in a sports stadium (Fig. 5.24), and the cone of light from a cinema projector.
- Car headlights on a foggy night; the beam is visible far ahead because water droplets scatter the light.
- A laser or a torch shone through milk diluted with water, or through a beaker of dilute soap solution.
- The blue of the sky and the red of the setting sun — sunlight scattered by the molecules and fine particles of the atmosphere.
Why the beam becomes visible: You see an object only when light from it enters your eye. A beam travelling across your line of sight sends you nothing — unless something turns part of it sideways. Suspended particles do exactly that: each one scatters a little of the beam in all directions, and the fraction that reaches your eye traces out the path of the beam. Clean air and true solutions have nothing big enough to scatter light, so in them the beam remains invisible.