Sight is a dangerously easy sense to take for granted. Other senses can get dulled sometimes, like our sense of smell when we’re sick, leading us to appreciate them more when they return fully. But, barring vision impairments, few things remind us not to take our sense of sight for granted.
However, when you think about it, sight is a pretty amazing thing. Somehow, our eyes take in information about the world around us and convert it into images that make sense. Not only that, but they do it in a way that matches other people’s visions of the world, so we can feel confident we’re all seeing the same stars, sunsets, and skylines.
How does this mysterious process work? Let’s find out.
First, Light Hits Your Eye’s Surface
As anyone who’s ever stubbed their toe in the dark knows all too well, light and vision go hand in hand. The brighter the light, the better we see (at least until the light becomes so bright that it strains our eyes).
We can still see some in low light, but the darker it is, the harder it gets. Our night vision isn’t as good as that of other animals. And, if there’s no light, there’s no vision.
So, the sight process begins when light reflects off an object in our field of vision, then reaches the eye.
First, that light passes through the cornea, the clear covering over the front of our eyes, which is covered by a thin layer of tears. If you’ve ever poked yourself in the eye, you’ve felt your cornea (and maybe even scratched it by accident).
The cornea bends light so that the light can move through the pupil—the dark part at the center of your eye. The pupil is actually an opening that lets light through, although the cornea covers it at the front. If it’s getting dark, your pupil grows wider to help more light pass through so that you can see better. And, if it’s bright, your pupil shrinks so as not too much light gets in.
The iris, or the colored part of your eye, is actually what changes the size of the pupil. There are muscles in your iris, believe it or not, which expand and contract to adjust pupil size.
Next, Light Reaches the Inside of Your Eye
Now, the light has passed through your pupil to the inside of your eye. This is the part of your eye that you can’t see when you look in the mirror.
The light first hits the lens behind your pupil. This lens is also connected to eye muscles, which change the shape of the lens so you can focus on objects depending on how close or far away they are. Much like a camera, your eyes can’t focus on both the foreground and background at the same time.
Behind the lens, light meets the vitreous body of your eye—a transparent, squishy gel that gives your eye its round shape. This gel is clear and enables light to easily pass through it, reaching the retina at the back of your eye.
The retina contains millions of nerve cells that are experts at sensing light. You might have heard these cells referred to as rods and cones before—they come in two different shapes. Cones give us the sharp images and vivid colors we see in bright light, while rods provide us with peripheral and night vision.
The retina’s cells gather these rays of light and turn them into electrical impulses that can be sent through the optic nerve to your brain.
Finally, Your Brain Interprets the Image
Now, what you’ve just seen has entered your brain in the form of electrical impulses, which get sent to the visual cortex in the rear of your brain. This part of the brain knows how to turn those electrical impulses from the retina into an image that makes sense to you. The visual cortex tells us what we’re looking at, and where it’s located in space. It’s the final step in the vision process.
As you can see, this process of decoding light into sight is fairly complicated, involving many steps and delicate parts that have to work in sync. However, it all happens so fast that, to us, it seems instantaneous.
Because human eyes and brains all work mostly the same way, we all see the world more or less the same. But many animals see a vastly different picture than we do when they look at something. For example, cats and dogs beat us at night vision by a long shot. Some animals can see ultraviolet colors that we can’t, while others can see in infrared. And, of course, there’s the mantis shrimp, which can see ten times more color than we can.
Look up from your phone or computer, and you just might have a newfound appreciation for what you see, now that you know how it works. And we’re covering all the senses here, so check back soon for answers to your questions about hearing, smell, touch, and taste!