Eye Function Experiment

Floating Finger

This week's experiment came from my niece, Lauren. She demonstrated some of the things she has been doing in science class. This is one that we did as kids, just for fun. Now I see the science side of it as well.

To try it, you will need:

• your fingers
• your eyes

This is a really simple one that shows some of how our eyes work. Start by pretending to be a mosquito. By that I mean to stick out your finger as if you are pointing at something. Put your hand to the end of your nose where your finger will stick out, like the long, thin snout of a mosquito. You can even hum a bit if you want to complete the image. While you are doing that, look at your finger. Yes, I know that it is not easy to look at it when it is that close, but that is part of the experiment.

What do you see? Two fingers! One on the right side of your nose and one on the left. Each finger you see is transparent, so although you can see the finger, you can also see through it. Why do you see this? It is because your eyes are seeing your finger from two different angles. You can see this better by closing one eye. Suddenly, one of the transparent fingers will disappear. The other becomes solid. Your brain is only getting one image, so things look normal. Now open both eyes and the two transparent fingers are back. Close the other eye. Again, you now see only one finger and again it is solid, but it is on the other side. With both eyes open, on the left side, one eye sees a finger, but the other sees whatever is behind the finger. The same thing happens on the right side, in reverse. As your brain puts the two images together, you see two transparent fingers instead of one solid one.

Now for the fun part of the experiment. Stop playing mosquito. (You can still hum if you want.) This time, you will need both hands. With your right hand, stick out your index finger, just as if you were pointing at something. Hold your hand on the right side of your face with your finger pointing to the left, so that the end of your finger is about three inches in front of your right eye. What do you see? A transparent finger, right? Keep that finger in place and do the same thing with your left hand in front of your left eye. Now you should see two transparent fingers, one in front of each eye.

Here is where things really get interesting. Slowly move your fingers toward each other. As the two transparent fingers cross, you will see something strange. Suddenly, you will see a solid, double ended finger floating between the two transparent fingers! What is going on?

Your finger looks transparent because one eye is seeing the finger and the other is seeing what is in the room behind the finger. Your brain puts the two images together, and you see a transparent finger. When the two images cross, you get a different result. One eye sees your right finger. The other eye sees your left finger, in the same place. Neither eye can see what is in the room behind the fingers, so the floating finger looks solid as your brain combines the two images. Keep moving your fingers together until they touch. Now, you should see two solid fingers, with a solid, double ended finger between them. Your brain, combining the two different images, has come up with a very strange combination, which as a child, I found very funny. It probably says something about my level of thinking that I still do. Have a great week!

Back to our Science Experiments index

All lessons are brought to you by The Teacher's Corner and Robert Krampf's Science Education Company.

Robert Krampf's Science Shows thehappyscientist.com