Imagine this: it’s early morning. No sounds. No smells. But open your eyes and, as long as there is light, there is always something to see. From the moment you wake up in the morning, until the moment you go to sleep, your eyes are taking in information and relaying it to your brain to interpret! Of all the senses, sight is the richest and most complex.
Look Closely
Sit a friend or family member down and look into one of their eyes. The first thing you’ll notice is that the eye sits in a hollow space in the skull and is protected by an eyelid and a bony eyebrow. As you gaze deeply into the eye, you look through the transparent, curved cornea to the donut-shaped ring called the iris, sitting in the center of the white of the eye. The iris is the part that can vary from blue, to brown to hazel and determine eye color. It’s also the part whose muscles cause the pupil, the dark circle in the center, to enlarge or contract.
It’s through the pupil that light rays enter the eyeball. The size of the pupil changes depending upon the light available. Just behind the pupil is the lens. It’s the transparent part of the eye which bends those rays of light and focuses the image on the back surface of the eyeball. How? With the help of muscles that actually change the shape of the lens!
Looking at something close up? The lens will become thicker. Admiring something that’s far away? Muscles will squeeze the lens, making it thinner so that you can see the image clearly. Remarkable, don’t you think?
Once the light travels through the lens, it must still travel through lots of clear jelly, called the vitreous humor, which makes up most of the eye. But, finally, the light makes it to the back surface, or retina, of the eye.
130 Million Light-Sensitive Cells
The retina, about the size of a postage stamp, is filled with two different kinds of light-sensitive cells — 130 million of them! Rods register shapes and respond to low levels of light. Cones, on the other hand, register color and only work in bright light (which is why colors become harder to see as it gets darker). Then, through optic nerves, these light-sensitive cells send information to the brain.
What about the images that are being communicated to the brain?
Remarkably, the images shining onto the retina and also being communicated to the brain are upside down! It’s the brain’s job, not the eye’s, to translate those upside-down images and interpret the information it receives into visual meaning that you can understand.
Why do people sometimes need glasses?
Sometimes the lenses in peoples’ eyes don’t properly focus light on the back of the retina. If an eyeball is too short, the image will fall behind the retina. People are then called far-sighted, because their eyes can focus on things far away but not close up. If, on the other hand, an eye is too long, people see things nearby but not far off and are called near-sighted. Either way, glasses or contact lenses can generally enable them to see much better!
Factoids
- You may think humans have good eyesight but imagine being an owl. An owl can see a mouse moving over 150 feet away with light no brighter than a candle!
- A cat’s eyes glow in the dark because of special silvery “mirrors” that reflect light, making it much easier for them to see in the dark.
- So-called “color-blindness,” in which colors such as green and red are hard to distinguish, affects about 1 in 30 people — and many times more men than women!
6 senses 