2026年10月9日 / Last updated : 2026年10月9日 桑子 研 Science

Making the Invisible Visible: A Laser and Fog Machine Light Show!

I’m Ken Kuwako, your Science Trainer. Every day is an experiment.

Point a laser pointer at a wall, and a bright red or green dot pops into view. But the path the light takes from the pointer to the wall? Normally, you can’t see it at all. The light is definitely traveling through that space, so why is it invisible? And what if there were a strange device that could whip up a cloud in your classroom in seconds and make that path of light appear? Wouldn’t you want to try it?

Normally, you can’t see the path of light

Recently, I tried viewing the path of light (the trail of a laser beam) in a class of seventh graders. If you simply use a laser pointer, you can’t see its path. So I used a chalkboard eraser to kick up a cloud of chalk dust along the laser’s route. With the room darkened, the beam suddenly shows up like this.

A word of caution: since you’re flinging chalk dust around, keep students well back so they don’t breathe it in. You can also do this with incense smoke. In that case, collect the smoke in a container such as an aquarium tank, then shine the laser through it and enjoy the light show. And one more very important rule: lasers can be dangerous to the eyes, so never point one at anyone’s face or eyes.

Why does dust or smoke make light visible?

We see something when light enters our eyes. A laser beam travels in a perfectly straight line, so in clear air, none of that light reaches the eyes of someone standing to the side. That’s why the path stays invisible.

But when chalk dust or smoke particles are in the way, the light hits them and bounces off in every direction. This is called light scattering. Some of the scattered light reaches our eyes, so the particles glow, and the path of the beam appears.

In fact, you can see the same thing in everyday life. When sunlight streams down through gaps in the clouds in great shafts, sometimes called “angel’s ladders,” you’re seeing light scattered by water droplets and dust in the air. A classroom laser is basically a miniature version of that scene.

I don’t want kids breathing dust… enter the fog machine

Honestly, I never felt great about having children inhale chalk dust. That’s when I started using a fog machine.

500W Fog Machine

A fog machine, the kind used for stage effects, makes clouds from nothing more than a 1:1 mix of glycerin and water. Keep one on hand, and you can fill a classroom with clouds in no time.

How does a fog machine make clouds?

The basic idea is simple: heat it into vapor, then cool it into tiny droplets.

First, a heater inside the machine warms the glycerin-and-water mixture until it turns into vapor. That vapor shoots out of the nozzle, hits the cooler air outside, and cools rapidly, turning into countless liquid particles too small to see individually. These tiny particles float in the air and look like a thick, billowing white cloud.

Natural clouds form in almost exactly the same way. Water evaporates from the ground and the sea, cools high in the sky, and condenses into tiny droplets. A mass of those droplets is a cloud. And a cloud that forms close to the ground? We call that fog. That’s where the fog machine gets its name.

So why add glycerin? Unlike water, glycerin doesn’t evaporate easily, so the particles hang in the air for a long time without vanishing. That’s why the cloud lasts much longer than it would with water alone.

A safer, more beautiful light show

Fill your classroom with fog and send a laser through it, and the beam looks far sharper than it ever did with chalk dust. Add mirrors, convex lenses, or glass, and you can watch light reflect, bend, and converge in three dimensions, right before your eyes. Seeing for yourself how light behaves, something you’d normally only find in textbook diagrams, is the real thrill of experiments.

Making the invisible visible is one of the great tricks of science. Next time you’re around light, try imagining what kinds of particles might be in its path, and take a closer look at the light around you.

By the way, a fog machine also works great with a vortex cannon!

煙が宙を舞う！フォグマシンで解き明かす「見える空気砲」の科学（回転の科学）

Inquiries and Requests

Science is full of wonder and fun, and I want to bring it closer to you! This site collects fun science experiments you can do at home, along with tips to make them work, all explained simply. Try searching around!
・The content of Kagaku no Netacho (the science idea notebook) has been published as a book. Details are here
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  • 『高校入試 分解問題集 理科』（学研）…難しい問題も小さな問題に分解することで、問題を解くことができます。そんな分解の技術が身につくように深く関わりを持って作りました。
    『大人のための高校物理復習帳』（講談社）…一般向けに日常の物理について公式を元に紐解きました。特設サイトでは実験を多数紹介しています。※増刷がかかり６刷となりました（2026/02/01）
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  • 『きめる!共通テスト 物理基礎 改訂版』（学研）…　高校物理の参考書です。イラストを多くしてイメージが持てるように描きました。授業についていけない、物理が苦手、そんな生徒におすすめです。特設サイトはこちら。

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