Exploring the Optical Illusion of Moving Images

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When I worked at NASA, I had the privilege of working with IMAX on one of their films.

One of the trips I took for that project was up to a projection booth to see how the films were loaded. It was a massive machine, carefully threading the long film strip.

Movies are made up of lots of still pictures in a movie strip, with each still picture in the sequence slightly different than the one before. Since those pictures move quickly through the film projector, they don’t seem like individual pictures, but instead appear to move.

Why is that? We’ll find out in today’s experiment!

This experiment teaches about the persistence of vision, which is when the human eye retains an image for a short period after it has disappeared. This lingering perception is due to the way our brain processes visual information.

This experiment feels like a magic trick, but it’s really science in motion!

How to make the Optical Illusion of Moving Images STEM experiment

Supplies you will need

For this experiment, you’ll need:

Scissors
Drinking glass (or similar size to draw a circle)
Single hole punch
Markers
Rubber bands (2)
White index card (2)
Glue stick

Supplies needed for the Optical Illusion of Moving Images STEM experiment

Before you start

Be sure to use a thick index card or cardstock, otherwise the rubber bands may rip the card.

Instructions

Here is how to do this experiment with your child:

Step 1: Cut index card into a circle

Use a glass or something circular and of similar size to draw a circle on two index cards.

Step 2: Draw a circle in one circle, a smiley face on the second circle

One one index card, draw a thick circle inside the circle you drew in step 1.

In the second index card, draw a thick smiley face inside the circle.

Cut out each circle so you are left with two circles, then glue them back to back.

A circle and smiley face draw on the index cards

Step 3: Cut holes in the sides of the card

Cut two holes across from one another, near the edges of the circle.

Step 4: Thread a rubber band in each hole, making a knot

Thread a rubber band through one of the holes you created to create a knot. Repeat with the other rubber band in the other hole.

Be sure the knots are secure and snug!

Rubber bands knotted through the index card holes

Step 5: Twist the bands

Twist the index card and rubber bands around until the rubber bands are wound up. You will have to play with the amount you wind up the bands to see what works best and produces the best results.

Step 6: Let the card spin!

Holding onto the rubber bands tautly, let the card go. The card should spin rapidly while you hold the rubber bands.

As the card spins, you should see the two sides start to blur together!

The speed of rotation is important. If it’s too slow, the eye can distinguish between the individual images. If it’s too fast, the images may appear blurry or indistinct. The optimal speed is when the images are blended seamlessly, creating a new, perceived image.

The STEM behind the Optical Illusion of Moving Images STEM experiment

This experiment teaches:

Persistence of vision
Coloring blending
Fine motor skills

How it works

Imagine you’re watching a movie. Each frame of the movie is a still image, but when they’re shown quickly in sequence, it looks like smooth motion. This is because your eyes can’t see each frame individually, so your brain blends them together.

The spinning card experiment is similar. Each side of the card is like a still image. When you spin it quickly, your eyes can’t see each side separately, so your brain blends them together. It’s like a mini-movie happening right in your hand!

It works by exploiting the human eye’s persistence of vision. This phenomenon is our brain’s ability to retain an image for a brief period after it’s gone.

This experiment is a simple yet effective demonstration of how our visual system works and how our brain processes information.

Persistence of vision

Persistence of vision is a fascinating phenomenon where our eyes retain an image for a short time after it’s gone. This happens because our brain takes a bit to process visual information. When we see something, light enters our eyes and hits the retina. The retina’s photoreceptors turn this light into electrical signals, which our brain interprets as an image.

Even after an object disappears, these photoreceptors keep firing signals for a brief moment. This creates the illusion of persistence of vision. How long this lasts depends on factors like the image’s brightness and duration, our age and eye health, and the surrounding light.

This phenomenon is essential for understanding motion. In movies, individual frames are shown quickly, but our eyes blend them due to persistence of vision, making it seem like smooth movement. This principle is also used in optical illusions, like the spinning card trick, where two images blend into one due to the rapid change in visual stimuli.

Coloring blending

Another variation of this experiment is, instead of drawing an image on each side of the card, color the card in different colors on each side.

For example: color one side of the card blue and the other side red.

When you conduct the experiment with that color combination, you should see purple! It’s another example of persistence of vision, but this time with color.

It’s a great way to show how primary colors can blend to make secondary colors.

Fine motor skills

Spinning the card requires precise hand-eye coordination and fine motor skills. This can help strengthen these abilities, which are essential for tasks like writing and drawing.