Stroboscope

Age

Elementary School, Middle School, High School

Format

Stage Show

Materials

   Stroboscope
   Standard AC outlet
   Stroboscope "disks"
   Standard-head screwdriver 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. The flashing strobe may encourage epileptic seizures - make sure the audience is aware of this before you present the demonstration so they may excuse themselves if necessary.

Preparation

Make sure that the stroboscope is plugged in and that the disk is securely locked on.

Demonstration

Demonstrate the disk to the audience.

Turn on the motor without the strobe to illustrate the blurring effect.

Turn on the strobe. You may wish to turn off overhead lights.

Adjust the speed so that they are synchronized.

Demonstrate other disks.

What to Say

Before we begin, I'd like to ask whether anybody here suffers from epilepsy. This experiment does contain a flashing lights, which can be a problem for some people.

Here's the disk that we will be demonstrating. You can see now what it actually looks like.

[Turn on the motor] With the disk spinning, our eyes blur the image together making it look like something else.

[Turn on strobe] With the strobe light on, we only see the disk for brief flashes. This causes it to appear still differently.

[Adjust the strobe speed] By adjusting the frequency of the flashes, we can change the image, making the disk appear to stop.

[Repeat the demonstration for multiple disks]

We are only viewing the disk at discrete moments in time. This is called temporal aliasing. Your mind is tricked into believing that these images all flow together, even though the disk is spinning between flashes.

Can anyone think of an example of this effect elsewhere in life? (motion pictures; spinning car tire appearing to move backwards)

This also illustrates the importance of choosing good samples in an experiment. If you were to live in a cave and come out only once a day at noon, you would be convinced that the sun never sets.

If you come out once every 23 hours, you may even believe that the sun moves backwards very slowly. During and experiment, scientists need to take samples frequently enough to understand what is actually occurring.

[Thank the audience and ask for questions]

Why It Is

We are only viewing the disk at discrete moments in time. This is called temporal aliasing. Your mind is tricked into believeing that these images all flow together, even though the disk is spinning between flashes.

Real Life Examples

Motion pictures.

A spinning car tire appearing to move backwards.