Key concepts
Optical illusion
The eye
Stroboscopic effect
Rotational speed

Introduction
Do you have to see it to believe it? You might want to rethink your strategy. Scientists now know that what we perceive can be very different from what is really there. Our brains are quite clever in helping us interact with the world, but they can be fooled. Try this activity, and you will find out how!

Background
We see with our eyes, but also with our brains. Our eyes register incoming light; electrical pulses transport the information to the brain, which processes it and informs us of what we see.

The area at the back of our eyes is called the retina. It is equipped with two types of light-sensitive cells: cones and rods. They send electrical signals to the brain when triggered. Cones are sensitive to color and suited for detail; they need bright light. Rods are sensitive to movement, shape and intensity changes; they are very sensitive to light.

Brain cells involved in processing visual information also have their own special jobs. Some are specialized to recognize movement and location as well as the rough outline or shape of what we see. These tend to work fast, at the expense of detail. Others specialize in recognizing the details of what we see.

Our visual system has evolved to serve us well. It is especially good at noticing patterns, even when information is missing. But this also makes the brain vulnerable to being tricked. Our visual perceptions can differ from what is really there. We call these optical illusions. In this activity you will use a rapidly flickering light to create some interesting optical illusions.

Materials

  • Eggbeater (A hand-rotary beater is preferred, but a handheld electric beater works as well. A beater with four blades works best for this activity. Use an adult's help with any electrical appliances.)
  • Steady light source such as light from a window or an incandescent lightbulb.
  • Rapidly flickering light source (Many computer screens have flashing screens and work well. MacBooks and many other recent computers do not work well, however, because they do not have flashing screens. Compact fluorescent lightbulbs, or CFLs, and LED lights also work as well.)
  • High table or counter if you use a computer as a light source
  • Chair that will allow you to sit eye-level with the screen (optional)

Preparation

  • If you are using a CFL or LED light source, make sure that you have a room where the CFL or LED lightbulb can be the only source of light, such as a closet or a room at night. (Have an adult help if any lightbulbs need to be changed or lamps moved.)
  • If you are using a computer as your light source, place your computer on a sturdy table or counter. Place a chair in front of the computer and adjust the height so that the computer screen is at eye level when you sit on the chair (or if standing is easier, that is fine, too). Set your computer screen with a solid, light-colored background, such as the blank, white screen of a document program.
  • If you are using an electric beater, be sure there is an outlet nearby for both light conditions—even if you have to change rooms.

Procedure

  • Begin in an environment with a steady light source (such as in natural light in front of a window or in a room with an incandescent lightbulb). Hold your beater in front of you, at eye level. Look through the blades (which should not be rotating just yet) at a wall or out the window. Can you tell how many blades your beater has?
  • Start rotating your hand-operated beater or turn the electric beater on and use with an adult's help. Begin with the lowest speed. Look through the blades at a wall or out a window. Can you still see the individual blades? Or do you see one blurry continuous shape? Why do you think this happens?
  • Increase the speed, step by step. Do you see a difference? Can you see that they turn faster? If so, how can you tell? Can you tell in which direction the blades appear to turn?
  • Stop your beater and move to a rapidly flickering light source (such as the older computer screen or a CFL or LED lightbulb). If you are using a computer screen, sit or stand in front of the computer so the computer screen is at eye level. Make sure your computer is set on a solid, light-colored background and hold your beater in front of you so the beater points toward your computer screen. If you are using CFL or LED light, hold your beater under the light source, preferably against a solid background, such as a blank wall. Start rotating your hand-operated beater or turn the electric beater on at low speed. Look at the computer screen or wall through the blades. What do you see now? Is it different from your previous observation? If you see individual blades, how many can you see? Can you tell in which direction the blades appear to turn or do they appear to be motionless? (Dimming the light might make the effect clearer.)
  • Increase the rotational speed of your blades by turning a little faster on your hand-operated beater or by moving up a step on your electric beater. Do your observations change as you move to a different speed?
  • Repeat the previous step until you can no longer increase the speed. You know the blades rotate faster when you increase the speed but do they also appear to do so? Do they always appear to rotate in the same direction? Can you explain your observations?
  • Extra: You just tested two different light sources, a steady light source and a flickering one. What other light sources can you find? What results do you expect from a television screen, the sky or a projector screen? Perform the test to find out. (Note that you should never look into a bright light, laser light or directly toward the sun—these could damage your eyes.)
  • Extra: If you are not sensitive to seizures, you can repeat the test with a strobe light as the background. Look for a strobe light generator on your computer. Sometimes you can even adjust the frequency (or the rate) at which the light flashes. If you can, play around with it. Can you make your blades appear to stand still or turn backward? (Note that strobe light can cause seizures, so be careful and stop immediately if you feel strange looking at the flashing light.)
  • Extra: Can you think of anyplace where knowing this effect could be handy? Could it help someone observe the speed at which the blades of a wheel rotate?

Observations and results
It is easy to count the blades of your eggbeater when the beater is still but it can become difficult once it starts rotating. Humans can see detail when the conditions are right. We see movement faster at the expense of detail.

With a steady light source, you probably perceived the fast-turning blades as a blurry shape, almost like a partially transparent solid object. When you switched to the computer screen or other flickering light source, the blades were visible again, even when they were rotating fast.

With a steady light source, nothing about the image is changing dramatically from one second to another. Our brain does not detect major changes, therefore the brain interprets the spinning eggbeater more as a solid object. With a flickering light source, however, your eye registers information only when the light flashes. The information coming from your eyes is changing dramatically between one "report" where the light flickered off and another where the light flickered on. In this case, our brains see the "appearance" of the blade as the most important thing. Therefore, you perceive the individual blades of the eggbeater. As the brain reconstructs images of the blades in different positions, it also concludes this is a moving object, which is what you perceive.

The tricky part is that the movement you perceived is not the real movement. Depending on the rate at which the light source flashes and the speed at which your blades spin, you might have perceived the blades rotating forward, backward or not at all. You know that the blades were always moving in the same direction, faster and faster as you increased the speed, but you perceived it differently. Your brain got fooled. What you perceived was an illusion. This illusion is referred to as the stroboscopic effect. Any rapidly flashing light (some sources suggest 15 flashes per second or more) can create stroboscopic effects. A steady light, such as from the sun or an incandescent lightbulb, will not create this illusion.

More to explore
How the Eye Works, from the Children's University of Manchester
How Strobe Light Makes Droplets Appear to Fall in Slow Motion, from Cornell College Funky Science
Set-in-Motion Science: Apparent Movement in Flip-Books, from Scientific American

This activity brought to you in partnership with Science Buddies

Science Buddies