Light shows the hidden particles

The idea

The scattering of light by particles is the Tyndall effect; it makes the path of a light beam visible in a colloid or a suspension, but not in a transparent solution.

A beam in a dark room

A fine beam of light enters a dark room through a small hole. You can see its path, shining in a straight line through the air.

Why can you see the path of the beam?

• Dust and smoke particles in the air scatter the light
• Light is always visible
• The air is a solution

Shine a torch through three liquids

Switch on the torch and shine it through each liquid, looking from the side. Where can you see the beam?

Tyndall effect

The scattering of light by particles is the Tyndall effect. It makes the path of a light beam visible in a colloid or a suspension, but not in a transparent solution.

It is named after the scientist John Tyndall, who studied the scattering of light by particles. You can also see it in the floodlights of a sports stadium.

Notes

The Tyndall effect is the scattering of light by particles. The beam’s path shows in a colloid or a suspension, not in a transparent solution.

Check yourself

A torch beam is shone through a salt solution. Its path is…

The scattering of light by particles is called the…

Light scattering occurs when light passes through a…

A beam of light enters a dark room through a small hole and its path is visible. What scatters the light?

• not visible — correct. Yes. A transparent solution does not scatter the light.
• bright and visible. That happens in a colloid or a suspension.
• visible only in the dark. The solution does not scatter it, in the dark or not.
• Brownian effect. That is not the name used here.
• Tyndall effect — correct. Yes.
• Solubility effect. Solubility is about dissolving.
• colloid or a suspension — correct. Yes.
• transparent solution. A transparent solution does not scatter light.
• vacuum with no particles. With no particles there is nothing to scatter it.
• The hole. The hole only lets the beam in.
• The walls of the room. The beam is seen in the air, scattered by particles.
• Dust and smoke particles in the air — correct. Yes.