The "Julia Child" of Kids' Hands-On Science
When I’m at a waterfall, I tell anyone near me how to get the illusion. They probably think I’m crazy, but then they do it and they’re amazed as well. (Or at least, they seem to be.) Since you’re probably not near a waterfall right now, I decided to hike to a waterfall in the Endless Mountains of Pennsylvania to make a video so you, too, can have this amazing experience, although I’m not sure if a video is as good as the real thing. But hey, life is about experimenting, no?
So when you’re done reading this, see if you can play the video below on a full screen. Stare at an imaginary spot in the middle of the falling water for the length of the video. Then shift your gaze right, to the rocks in the video or wait for the freeze frame of the falls, or even try looking at something that’s not moving in your room. You should see the image rising. If you don’t, you should probably take a hike to your nearest waterfall and try again.
The receptor cells (the nerves in your eyes) that perceive motion get tired after a while if you are looking at falling water. When you stop looking, the receptors that perceive upward motion are not tired, so they fire away. As a result you perceive motion in the opposite direction although it’s not really happening.
The first person to observe this illusion was the famous German physicist Hermann von Helmholtz (1821-1894). In 1860 was riding in a train, staring out the window at the scenery passing by. When he shifted his gaze to the inside of the railroad car, it appeared to be moving, but in a direction opposite to the view of the landscape.
Next time you go to a waterfall, share your knowledge with other bystanders. They may even thank you.
This classic illusion book has some amazing tricks you make on yourself! Discover the incredible shrinking sugar cube, make movie style sound effects, and fool grown-ups with flavorless coffee.
Fooling your senses can blow your mind.
Vicki is a member of iNK's Authors on Call and is available for classroom programs through Field Trip Zoom, a terrific technology that requires only a computer, wifi, and a webcam. Click here to find out more.
Tomorrow Andrea Warren will introduce you to 11-year old Billy who went to work as a trapper, trail guide, scout, and marksman in order to support his mother and six siblings. Can you guess the name by which we know him today?