See your visual system turn things upside down, in this Moment of Science.
This is a simple visual experiment. To do it, you need two three-by-five index cards.
Take one of the index cards and make a tiny triangle of holes in it with a straight pin. Make the three holes about a sixteenth of an inch apart.
Take the other index card and punch just one pinhole in it.
Now take the card with the triangle of pinholes, and hold it very close to your eye. Hold the other card, the one with the single pinhole, about four inches in front of the first one. Face a strong light and look through the three holes at the single hole.
You will see your triangle of pinholes upside down. Try it.
The holes in the two cards cast three thin beams of light into your eye, making three dots on your retina.
Under normal circumstances, you retina gets a focused, upside-down image projected by the lens of your eye. Your nerves and brain in effect turn that image over to make it right side up.
But in this experiment, the card with the three holes is so close to your eye that the eye lens can't focus the three dots into an image. The rest of your visual system, operating as usual, inverts the triangle of light on your retina anyway, and you see the pinhole pattern upside down.
Once again, make a tiny triangle of pinholes in one card; hold that card very close to your eye. Make just one pinhole in a second card; hold it about four inches in front of the first card while you look toward a strong light. You'll see the triangle upside down. It's a very striking effect; we hope you'll try it.