Science Puzzle
Why Do You Need Two Eyes?
Hold a pencil in each hand at arm's length and bring them together so their tips touch. With both eyes open, it's easy. Now close one eye and try again, quickly. Most people miss, often by a centimetre or more.
Closing one eye doesn't make the world blurry or dark. Why does it make this simple task so much harder?
The Answer
Your two eyes give your brain two slightly different views, and comparing them is one of the best ways it has of judging how far away things are.
Your eyes are about 6 centimetres apart. Hold up a finger and look at it with one eye, then the other: it seems to jump sideways against the background. The closer the finger, the bigger the jump. Without you noticing, your brain measures that difference all the time and turns it into a sense of depth. It's called stereo vision.
Close one eye and that clue disappears. You still have others, such as how big things look, how things shift as you move your head and which things block your view of which. They're good enough for most of daily life, but not for lining up two small tips in empty space.
Predators such as cats, owls and eagles have eyes facing forwards, with views that overlap a lot, which helps them judge the distance to their prey. Many prey animals, such as rabbits, have eyes on the sides of their heads. They're worse at judging depth but can see almost all the way round to spot danger. 3D films work by sending a slightly different picture to each of your eyes.
The principle: Binocular vision. Two eyes set apart see an object from slightly different angles, and the brain uses the difference between the two images to judge distance.