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The Wagon Wheel Effect Optical Illusion

Summary

Active Time
10-20 minutes
Total Project Time
10-20 minutes
Key Concepts
Optical illusions, frame rate
Credits
Ben Finio, PhD, Science Buddies

Introduction

Have you ever seen a video where it looked like a car's wheels were spinning backwards even though the car was driving forward? This is called the wagon wheel effect, named after old movies where it looked like wagon wheels were spinning backwards. Even though you cannot see the effect in person, it can show up in videos of spinning objects. Find out why in this project and make your own videos of the wagon wheel effect!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Materials

  • Smartphone or camera
  • Spinning object like a ceiling fan, box fan, or power drill. If you use a drill, you will need a piece of masking tape to put on the drill bit so you can see it rotating.

Prep Work

If you do not know how to use the manual settings on your camera, ask an adult for help. If you use the automatic settings, your video may be too blurry. You can use the manual settings to increase the camera's shutter speed, which decreases the exposure time for each frame of the video. This will make the video less blurry. You can also manually select the frame rate for your video. Most phones can shoot at frame rates of 30 or 60 frames per second (fps), and may have slow-motion video options at 120 or 240 frames per second. 30 frames per second is a good place to start.

Instructions

  1. Turn your fan or drill on the lowest speed setting (for some drills, the speed may be controlled by how hard you squeeze the trigger). Watch carefully to see whether the fan blades or drill bit are spinning clockwise or counterclockwise.
  2. Record a video of the spinning fan or drill and then watch it.
    Think about:
    Which way are the fan blades (or drill bit) spinning in the video?
  3. If the video is too blurry and you cannot clearly see the fan blades (or piece of tape on the drill bit), increase your camera's shutter speed and try again.
  4. Increase the fan's (or drill's) speed and record a new video.
    Think about:
    Which way does it look like the blades (or drill bit) are spinning this time?
  5. Record a new video for each speed setting for your fan or drill. If needed, continue to adjust your camera settings.
    Think about:
    Can you find a combination of speed and camera settings where it looks like the fan (or drill) is spinning backwards? This is the wagon wheel effect!

What Happened?

It might take a few tries, but you should be able to make your own video of the wagon wheel effect where it looks like the fan blades (or drill bit) are spinning in reverse. Read the Digging Deeper section to learn more about how the wagon wheel effect works.

Digging Deeper

To explain the wagon wheel effect, it helps to think about a clock with a second hand. Imagine what would happen if you took a picture of the clock once every 59 seconds, just before the second hand had completed one full rotation, and then stitched those pictures together to make a video. It would look like the second hand was moving backwards. If you took a picture exactly once every 60 seconds, it would look like the second hand was frozen in place. If you took a picture once every 61 seconds, it would look like the second hand was moving forward, but very slowly.

This is exactly what happens with the wagon wheel effect. A video is really a series of still images, played back quickly enough that the human brain perceives continuous motion. Your camera records video at a certain frame rate, or number of frames per second. Depending on the frame rate and the rotational speed of the object you're filming, this can result in apparent motion that is different from the object's actual motion.

Three rows, each consisting of five clocks with second hands only.

The first row shows the second hand moving backward by 1 second for each clock, with 59-second gaps indicated between the images. The second row shows the second hand in the same position on each clock, with 60-second gaps indicated between the images. The third row shows the second hand moving forward by one second for each clock, with 61-second gaps indicated between the images.

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For Further Exploration

  • Can you find the exact settings to make it look like your fan blades (or drill bit) are frozen in place and not spinning at all?

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