When a Flashing Light Shows More
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 appropriate 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.
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.
More to Explore
Sabine De Brabandere, PhD, Science Buddies
Science Buddies |
Optical illusion, the eye, stroboscopic effect, rotational speed
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