You and Your Students!
Vicki Cobb, Education World Science Editor
This experiment will help students understand fog “clearly.”
Physical Science, Light
Setting the Scene (Background)
Real fog is made of microscopic droplets of water suspended in the air. Fog appears to be white because each droplet acts like a tiny mirror and reflects any light that strikes it. If you look at a beam of light -- such as a car headlight -- from the side when there is no fog, you will not see the beam in the air. But when a headlight shines into a fog, you can clearly see the shape of the beam. The light that is reflected off such suspended particles is called the Tyndall Effect.
Act I: Experience the Tyndall Effect
The Tyndall Effect doesn't only show up in air. You can see it in other transparent materials, including water. Solutions -- salt water or sugar water, for example -- do not show the Tyndall Effect; in those solutions, salt or sugar molecules are suspended in water. Milk, however, does not dissolve in water. Instead, tiny milk droplets are suspended evenly in the water forming a mixture called a colloid. Put a few drops of milk in a glass of water. Shine a pen flashlight beam through the glass. Look at the beam from the side and you will see it as the light reflects off the tiny, suspended milk droplets. Shine a light in salt or sugar water and the beam is not visible.
Act II: Make Your Own Fog
Oil droplets alone can look like fog. In fact, there are fog machines that spray a kind of mineral oil to produce fog on stage for theatrical effects. You can see what it looks like by spraying a short blast of cooking oil in front of a light. (Just be careful to spritz it outside or over the sink so the oil doesn't get on fabrics or hard-to-clean surfaces.)
Behind the Scenes
Fog particles must be small enough to stay suspended in the air. In real fog, water molecules condense around dust particles in the air. Most special-effects fog machines produce a "fog" made from "fog juice" -- a mixture of a chemical called glycol and water. Glycol is a clear, syrupy liquid that is used as an anti-freeze in car engines and is sprayed on airplane wings to de-ice them in winter. When glycol is heated, it becomes smoky much as burnt cooking oil is smoky. A fog machine pumps the "fog juice" through a space where it is heated. This changes the liquid into a gas, which quickly expands. When a gas expands in a closed space, the pressure increases. The hot gas is now released into the air through a nozzle. Moisture in the air condenses around the hot gas molecules to form a fog.
Glycol is used for special effects instead of cooking oil because it has no smell and it rapidly evaporates to clear the air for the next scene.
Article By Vicki Cobb
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