Strobe Light Fan

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Light, Motion

Alternative titles: Freezing a Spinning Fan

Summary

When a spinning fan is illuminated by a strobe light, the blades can appear frozen, move slowly forward, or even seem to spin backward. This demonstrates the stroboscopic effect, where flashing light interacts with periodic motion.

Procedure

Place a desk fan or ceiling fan in a dimly lit room.
Turn on the fan at a steady speed.
Shine a strobe light directly at the fan blades.
Adjust the strobe frequency:
1. When the strobe rate matches the rotation speed, the blades appear frozen in place.
2. When the strobe rate is slightly faster, the blades appear to move slowly forward.
3. When the strobe rate is slightly slower, the blades appear to move slowly backward.
Try different fan speeds to observe changes in apparent motion.

Links

Spinning Fan with Strobe Light - PHYSICS EVERYWHERE !:

Motion - Strobe Light and Fan Demo Explained (READ WARNING BELOW!) - Jon White:

📄 Measuring the Speed of Moving Objects with Stroboscopic Photography - Science Buddies: https://www.sciencebuddies.org/science-fair-projects/project-ideas/Photo_p003/photography-video/measuring-the-speed-of-moving-objects-with-stroboscopic-photography

Variations

Place a piece of tape or mark on one blade tip to make the effect easier to observe.
Try with fans of different sizes (small desk fan vs large ceiling fan).
Record the effect with a phone camera for comparison with real slow-motion video.

Safety Precautions

Do not put hands near moving fan blades.
Avoid prolonged exposure to the strobe light as it may trigger discomfort or seizures in sensitive individuals.
Keep the strobe light at a safe distance from the fan.

Questions to Consider

Why does the fan appear frozen when the strobe matches the blade speed? (Because each flash shows the blades in the same position, creating the illusion of stillness.)
Why can the fan appear to spin backward? (If the strobe flashes slightly slower than the rotation speed, each flash shows the blades a little ahead, giving the illusion of reverse motion.)
How is this similar to the “wagon wheel effect” in old movies? (Both are caused by stroboscopic sampling of motion at discrete time intervals.)
What real-world uses are there for strobe lights besides demonstrations? (Studying vibrations, measuring machine speeds, and in entertainment lighting.)