Strobe Light Circular Saw

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★★ Requires a more experienced teacher
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Light, Motion

Alternative titles: Freezing a Spinning Saw Blade

Summary

A rotating circular saw blade can appear to stand still, move slowly forward, or even move backward when illuminated by a strobe light. This stroboscopic effect occurs when the strobe frequency matches or nearly matches the blade’s rotation speed.

Procedure

Place a circular saw (plugged in but not cutting) in a safe demonstration area.
Darken the room enough so the strobe light is effective.
Turn on the saw so the blade rotates freely.
Shine the strobe light directly at the spinning blade.
Adjust the strobe frequency:
1. At certain frequencies, the blade will appear frozen in place.
2. If the strobe rate is slightly faster or slower than the blade rotation, the blade appears to move slowly forward or backward.
Discuss how this is an optical illusion created by timing between the flashes and the spinning blade.

Links

🎞️ STROBE LIGHT SCIENCE - Minuteman High School: https://www.facebook.com/watch/?v=5652889581458209

🎞️ Strobe Light Wood Turning! - pswii360i (similar demonstration with lathe): https://www.reddit.com/r/gifs/comments/4wupfy/strobe_light_wood_turning/

Variations

Mark one tooth of the blade with chalk or paint to make the effect easier to see.
Try the same effect with a fan or bicycle wheel for a safer classroom demonstration.
Record the effect with a camera to show “aliasing” of motion.

Safety Precautions

Extreme caution: a circular saw is dangerous even when not cutting.
Only a trained adult should operate the saw.
Keep the saw in a stable, secure position—never bring hands near the spinning blade.
Use protective eyewear.
Consider using a safer alternative (e.g., a box fan, drill-powered disc, or toy propeller) for classroom demonstrations.

Questions to Consider

Why does the saw blade appear still under the strobe light? (Because the strobe flashes in sync with the rotation, showing the blade in the same position each time.)
Why can the blade appear to spin backward? (If the strobe rate is slightly slower than the blade rotation, each flash shows the blade a bit ahead, creating the illusion of backward motion.)
How is this effect related to movies and video of wheels appearing to spin backwards? (Both are stroboscopic effects caused by sampling motion at discrete intervals.)
Why is using a safer substitute (like a fan) better for classroom demonstrations? (It eliminates the risk of severe injury while showing the same optical principle.)