How Our Eyes See Everything Upside Down

The human eye is the second most complex organ in our body after the brain. It's amazing that something so small (the size of a ping pong ball) can have 2,000,000 working parts!

Materials

Piece of a carton box
Pin 

Directions

The human eye is the second most complex organ in our body after the brain. It's amazing that something so small (the size of a ping pong ball) can have 2,000,000 working parts! The eye gives you the sense of sight, allowing you to perceive more about the surrounding world than with any of the other four senses. It allows you to see and interpret the shapes, colors, and dimensions of objects. However, do you know that the image created on the retina is inverted? If yes, have you ever wondered how you could get more direct evidence of that fact? This activity will explain how the image on the retina in created and why is inverted. 


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Hold a pin head in front of your eye in a position you can see it clearly. Start moving it closer towards your eye. Can you see any difference in image quality when you do it? What does happen when the pin is just in front of your eye? Can you still see it? In the next step use a brightly illuminated pinhole (a piece of a carton box with a tiny hole) as a source, held about 10 cm from the eye. Hold a needle head between the pinhole and the eye. In this case the pin is less than a focal length from the eye, so it cannot form a real image on the retina. It casts a shadow on the retina, which looks `wrong side up'.

Explanation

The cornea-lens system of human eye behaves like a double convex lens. This lens allow us to create an image of the object  which size, orientation and location depends on the distance between the object and the lens. If the object (for example pin) distance is between f (focal-length) to 2f of the lens than the real, inverted and larger then object image is created on the other side from the object. If the object distance is farther than 2f of the lens than the real, inverted and smaller then object image is created. When we look at the different objects they are typically located at a point in space more than 2f from the "lens," then what is projected into the retina is inverted, reduced in size, and real image. The reduction in the size of the image allows the entire image to "fit" on the retina. The fact that the image is inverted poses no problem too. Our brain has become quite accustomed to this and properly interprets the signal as originating from a right-side-up object. If the object distance is less than the f of the lens than the virtual, not inverted, larger then object image is created and we can’t see it. However, if we placed a carton box with a tiny hole, this hole will serve as a point source of a light which illuminates our needle and casts a shadow on the retina, which looks `wrong side up' because the brain turns this shadow upside down.