Now You See It, Now You Don't

What we call light is simply a narrow band of electromagnetic radiation that our eyes are sensitive to. This radiation enters our eyes and is conveyed to the brain by the process we call sight. While the mechanics of seeing is quite complex, the process of seeing is, in a different sense, quite extraordinary. Here are two examples.

Have you ever used a telescope to view a distant object and realized that the image you are seeing is upside down? A telescope with convex lenses creates an upside down image. Your eyes do the same thing. As light enters your eye, it passes through your cornea and is focused by your lens onto the retina, which contains light-responsive cells called rods and cones. Because it works much in the same way as a telescope, the image projected on your retina is upside down. The optic nerves in the back of your eyes conveys this upside down image to your brain. But when you look at your cat, he's not walking on the ceiling. Thankfully, your brain does the switch for you, and flips the image.

Each of our eyes has a blind spot, a place on our retinas about the size of a pinhead where there are no rods or cones. Our blind spot is the place where our optic nerves exit the eye and connect to our brains. But we don't usually notice this blind spot. That is because our brains fill in the information for us. We think we see what we should be seeing. The trick, of course, is that as we move and focus our eyes, the blind spot is a moving target. Our brains can make a pretty good guess as to how to complete the picture of what we are looking at. To see your blind spot, follow the link to the larger image of the image on the right. Close your right eye and focus your left eye on the purple soccer ball. Now slowly move your head closer or farther from your computer screen. Can you make the orange soccer ball disappear? Then you've found your blind spot.

About the Author

Gene Mascoli, JD

Gene MascoliGene Mascoli is a founder and publisher of He holds a J.D. degree from the University of Santa Clara and a B.A. in English. In 1997 Gene launched, an online science education portal where he brought together his love of writing with his interest in the sciences. Gene collaborated with David Gamon on the popular digital book “The Internet Guide to NASA on the Net” and has also produced two popular science CD-ROMs on astronomy and space science.