The "Tyndall effect" appears in the cloudy sky of Beijing. Why does light have a shape?

On the morning of April 20, affected by the cold front, Beijing experienced strong winds and cooling weather, accompanied by blowing sand in some areas. The sky was no longer clear and transparent as in the previous few days, and the sun shone through the thick clouds, forming a "Tyndall effect".

What is the Tyndall effect? ​​What causes this weather phenomenon?

Copyrighted images from the gallery, unauthorized reproduction, please contact the original author

When a beam of light passes through a colloid, a bright "pathway" can be observed in the colloid from the perpendicular direction of the incident light. This phenomenon is called the Tyndall phenomenon, also known as the Tyndall effect. The Tyndall phenomenon was first discovered by British scientist John Tyndall in 1869. This phenomenon can also occur when light passes through clouds, fog, smoke and dust.

The Tyndall phenomenon can be seen everywhere in daily life. For example, people often say "sunshine shines on the earth", and the "sunshine" here refers to the Tyndall phenomenon; when people stroll along the forest path and look up at the sky, the sunlight pours down through the gaps in the leaves and hits people in beams. This is also the Tyndall phenomenon.

This phenomenon is formed because clouds, fog, and smoke are all colloids, and the dispersant of these colloids is air, which are aerosols; there are also liquid sols - dispersed systems with liquid as dispersant, such as protein solutions, starch solutions, etc.; and solid sols - dispersed systems with solids as dispersants, such as colored glass.

Of course, these phenomena can also be observed in the laboratory through simple experiments. Take a clean test tube with a larger diameter, add about 1/3 of the volume of water, clamp the test tube with a test tube clamp, and use a laser pen to illuminate it from the test tube mouth in parallel. No obvious phenomenon is observed in the upper space of the test tube and in the water. Heat the test tube to make the water boil, and the generated water vapor will escape. At this time, illuminate the test tube again, and a bright "pathway" is observed in the upper space of the test tube, and there is no obvious phenomenon in the water. Remove the alcohol lamp, and as the water vapor slowly condenses, it is found that the bright "pathway" gradually dims until it disappears.