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When a Flashing Light Shows More


Key Concepts
Optical illusion, the eye, stroboscopic effect, rotational speed
Sabine De Brabandere, PhD, Science Buddies


Do you have to see it to believe it? You might want to rethink your strategy, as scientists now know that what we perceive can be different from what is really there. Our brain is quite clever in helping us interact with the world, but it can get fooled. Of course, you need to see this to believe this. Try this activity, and you will find out!

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.


We see with our eyes, as well as with our brain. Our eyes register incoming light. Electrical pulses transport the information to the brain. The brain processes it and informs us of what we see.

It all starts in the eye. The back of our eye – also called the retina - is equipped with two types of light-sensitive cells: cones and rods. They send electrical signals to the brain when triggered.  The cones are sensitive to color and suited for detail. They need bright light. The 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. This 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 flashing or strobing light to create some interesting optical illusions.


  • Egg beater; hand rotary is best, but a handheld electric beater works as well. A beater with 4 blades works best for this activity.
  • A flickering light source.
    • Many computer screens have flashing screens and work well. MacBooks and other recent computers do not work well, because they do not have flashing screens.
    • CFL (compact fluorescent light) and LED lights work as well.
  • If you use a computer as a light source, you will need a high table or counter on which to put your computer, as well as a chair, preferably one with adjustable height.


  1. If you are using a CFL or LED light source, you can immediately move to the procedure and use sunlight as your steady light source.
  2. If you are using a computer as your light source, put your computer on a sturdy table or counter. If you are using an electric beater, be sure there is an outlet nearby.
  3. Place the 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. If standing is easier, that is fine, too.
  4. Set your computer screen with a solid, light-colored background, like the blank, white screen of a document program.


  1. 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?
  2. Start rotating your hand-operated beater or turn the electric beater on. Start with the lowest speed. Look through the blades at the wall or out the window. Can you still see the individual blades? Or do you see one blurry continuous shape? Why do you think this happens?
  3. Turn up 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?
  4. Stop your beater and switch to a flickering light source. 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. Start rotating your hand-operated beater or turn the electric beater on at low speed. Look at the computer screen 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.
  5. 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?
  6. 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 rotate faster? 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 light source. 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 or directly toward the Sun, as these could hurt your eyes.

Extra: If you are not sensitive to seizures, you can 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 backwards? 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 any place 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 egg beater when the beater is still, but 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 egg beater more as solid object. With a flickering light source, 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 egg beater. 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 (15 flashes per second or more) can create stroboscopic effects. A continuous light, like from the Sun, will not create this illusion.

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