You may not be old enough to have a driver’s license, so you cannot drive a car. But there is one vehicle that you do get to drive every day. It’s your body. Your brain tells your muscles what to do, and they respond. Sensors like your eyes, ears and nose bring information to your brain, so your brain can decide what to do next. The fascinating thing is that your body is one of the best vehicles ever built. Let’s look at some of the technology that makes your body so amazing.

Eyes
One way to understand how amazing your eyes are is to compare them to a digital camera. Let’s say you have a digital camera that has five megapixels of resolution. This means that there are 5 million little light sensors inside the camera. In each of your eyes, there are 120 million rod cells. That’s 24 times more than the digital camera!

The rod cells give you black-and-white vision, and your night vision. Your color vision comes from about 6 to 7 million cones inside your eye. All the cones are packed together in a tiny area called the fovea. The rods are spread out over the back of the eyeball in a larger area called the retina.

The rods in your eye are much more sensitive to light than a digital camera is. A digital camera cannot “see” at night unless you use a flash. The human eye is so sensitive at night that we can see a single candle that is 10 miles away. How can the rods be that sensitive to light? They use a chemical called rhodopsin. This chemical is very sensitive to light. If you go out at night (or if you sit in a dark room), it takes about 30 minutes for the rhodopsin to build up in your rods. When a photon of light hits a molecule of rhodopsin, the molecule changes and sends a signal to your brain. Your brain can actually see single photons of light.

A Carrot a Day … To make rhodopsin, your body needs vitamin A. If you don’t get enough vitamin A, your night vision becomes poor. This is why you might have heard that you need to eat carrots to improve your eyesight – carrots contain vitamin A.

The funny thing about your night vision is that you cannot see anything that is straight in front of you. That’s where your fovea is, and your cones cannot see at night. Cones need a lot of light, just like a digital camera does. In order to see things directly in front of you at night, you have to look a little to the side. Or you have to shine a light at it.

Your fovea has three kinds of cones: red, green and blue. If you were to look at 100 cones in your eye, 64 of them would be sensitive to red light, 32 to green light and four to blue light. This means that people “see” red more intensely. That may be why we paint our fire trucks and stop signs red.

When you use a digital camera, it takes the whole picture of the scene in one shot. Your eyes don’t work that way. Your fovea only sees a tiny part of the scene. Without you realizing it, your brain flicks your eyes all over the scene and creates a single image in your head from dozens of little patches. These are called saccadic eye movements.