How We See
Our eyes take in light. Obviously, we can't see in a completely dark room. (A lot of bumped shin bones can attest to that.)
The pupil is the black circle in the middle of our eyes. The iris is the colored part that surrounds it. The pupil can expand or contract depending on the amount of light there is. In environments with very low light, your pupil expands to take in more of what light is available. In environments that are very bright, the pupil contracts – or shrinks – in order to allow less light in.
This light enters through the pupil. After this, it passes through the lens. The lens focuses the light onto the retina. Your retina is a membrane that lines the inner surface of the back of your eye. It's extremely sensitive. It has millions and millions of what are called photoreceptors.
How Our Brain Perceives
What do these photoreceptors in the retina do? They convert the light that's focused on the retina into electrical impulses. These impulses are conveyed by the optic nerve and into our brains. The brain then interprets the information and gives us an image.
This is happening every instant of the day: the retina transmits 10 million bits of information per second along the optic nerve.
Is it true we see upside down?
In a way, yes. Obviously, the light coming into our eyes is entering the right way, but that light is bent by the curved shape of our eyeballs. The retina receives that light upside down. It's up to the brain to reinterpret the information it's getting right-side up. The visual information we get is “flipped” in the process before information is sent to our brain, but the brain corrects it so we perceive the world properly.
How far can the eye see?
On a clear day with perfect conditions, a healthy eye can see further than the horizon. This is why you'd be able to see ships disappear over the curve of the earth. But it really depends on how big the object is. In perfect conditions, you could see a candle flickering at least 14 miles away – if the candle or you were elevated enough to see past the horizon.
When you look up at night, we see hundreds of stars. The oldest object we can see in the night sky with the naked eye is a whole other galaxy, the Andromeda Galaxy. Light takes time to travel, so the light our eyes are getting from that galaxy is two million years old. Light can travel 5.88 trillion miles in a year, so if you multiply that by two million, that's how distant the Andromeda Galaxy is.
In other words, there's no real limit to how far we can see – the object just needs to be big enough.
Can people have better than 20/20 vision?
There are people with 20/10 vision, yes. This means that if someone with 20/20 vision had to be 10 feet away from an object to tell what it is, a person with 20/10 vision could tell from 20 feet away.
Someone who's legally blind is someone with a visual acuity of 20/200. This means they'd have to be 20 feet away to see an object that someone with 20/20 vision can see from 200 feet away.
Can people really have different color eyes?
Yes. This is called heterochromia. It has to do with the distribution of melanin in the eye. Actress Mila Kunis has heterochromia – her left eye is green and right eye is brown.
Contrary to popular belief, rock star David Bowie didn't have heterochromia – he suffered an injury in a fight when he was young. He avoided losing his vision, but the pupil in one eye is permanently dilated – this led the star to have depth perception and other visual issues throughout his life.
Do we really have a hole in our vision?
Where the optic nerve connects to the eye, there's a small blind spot in the retina. Our brains use other visual information to create a fuller picture than what we actually see. Part of this is that each eye can see where the other eye's blind spot is. Even when we close one eye, the brain uses other information to fill our vision in – remember that the eye takes hundreds of images per second. That's a lot of extra information to help fill in that retinal blind spot. (By the way, even though we can see hundreds of images per second, TV screens and monitors often operate between 50-90 frames per second because this is all we need for us to perceive “smooth” motion).
Our eyes are truly amazing! Take care of them so they can continue to help you see and experience the world around you. Schedule an appointment with Asheville Vision Associates today.