When a beam of sunlight enters a room, the light waves interact with the dust particles in the air. The dust particles act as obstacles that scatter the light in all directions. This scattering of light is what causes the dust particles to become visible.
The amount of scattering depends on the size of the dust particles and the wavelength of light. Smaller particles scatter light more effectively than larger particles. This is why the dust particles in a beam of sunlight appear to be moving rapidly, even though they are actually stationary.
The Tyndall effect is also responsible for the blue colour of the sky. The molecules in the Earth's atmosphere are very small, so they scatter blue light more effectively than other colours. This is why the sky appears blue during the day.
The Tyndall effect is a common phenomenon that can be observed in many different situations. It is a beautiful example of how light interacts with matter.
