20 Interesting Facts About Eyes » The Unclouded Path

Eyes are incredibly complex organs, so complex that each of us has a set that’s unique. And since they’re such a prominent feature of our faces, they’re a significant aspect of culture. But even being such familiar features, there are a lot of myths out there about them. So, this 20-fact post will be about dispelling some of those myths and about some very interesting facts about eyes.

1. Each eye has a wide field of view.

It’s so wide that a lot of it is actually blocked because of our facial features. Each eye can see roughly 30° up (brow in the way), 70° down, 45° toward the nose (nose in the way), and 100° to the outward side, all relative to the center of focus. With both eyes (binocular vision), the field of view is roughly 200° horizontal and 135° vertical.

Interestingly, the eye has a blind spot that’s near the center of its field of view. That’s because the retina has a spot where the optic nerve exits the eye. That spot has no light-detecting cells. It’s at around 15° outward, 1.5° down for each eye, and the area is around 7.5° high and 5.5° wide. You can actually see it for yourself by covering one eye, focusing on some point, and moving a small (but very visible) object like the eraser tip of a pencil to the blind spot’s location. It will seem to disappear.

2. Eyes aren’t spherical.

The “eyeball” is actually more like a sphere with a smaller dome in the front merged together. That smaller dome is the cornea, and it helps with focusing light into the eye. In fact, the protruding dome shape of the cornea is how light from the sides of the eye can enter it, giving you peripheral vision at those angles. You can see this effect in the following picture. The picture shows the eye from 90° to the side. You can still see the pupil from this angle because of the optical properties of the cornea. That means light from this angle is still able to enter the eye.

Photo of someone’s eye from the side. Note how you can still see the pupil, which means light can enter the eye from this angle. Source: Paul Savage (https://en.wikipedia.org/wiki/File:Mairead_cropped.png)

Even the spherical part of the eye can sometimes be not quite spherical. Some cases of near-sightedness are due to it being too long. Likewise, some cases of far-sightedness are due to it being too short.

3. You have over 100 million rod cells in each eye…

Rods are one of the two types of photoreceptor cells you have in your eye. They can’t sense color like the other kind of photoreceptor cell, cones. But since they’re mostly at the outer edges of the retina (back of the eye), they’re responsible for your peripheral vision. You can notice this by holding something colorful near the edge of your peripheral vision. The colors will appear to be very dull or even be in grayscale.

4. …but only about 6 million cone cells.

You have a lot less cones than rods, but they’re very important, too. They are the cells that give you your color vision, and they can see finer details than rods. They’re mostly in the center of the retina, in a region called the fovea centralis, which has no rods. This dense clustering of cones gives you sharp color vision in the center of your field of view.

5. Humans have three types of cone cells…usually.

They’re the S-cones, M-cones, and L-cones. The letters stand for short, medium, and long, and they indicate what wavelengths of light they’re sensitive to. S-cones are sensitive to wavelengths around 420 nm (blue/violet), M-cones are sensitive to wavelengths around 530 nm (green), and L-cones are sensitive to wavelengths around 560 nm (red/yellow). These three cones are why humans have trichromatic vision. With these cones, humans can distinguish about 1 million different colors.

Some people have a mutation that gives them a fourth type of cone that is between the M- and L-cones in terms of color sensitivity. These people thus have tetrachromatic vision and can distinguish about 100 million different colors. This mutation seems to be more common in females, likely because two genes for cone cell photoreceptor proteins are on the X chromosome. Although we don’t have a firm idea of how many people have this mutation, one estimate suggests that around 15% of females worldwide could have it.

Interestingly, the three cones’ range of sensitivity extends a bit into the ultraviolet range. You normally can’t see ultraviolet light because the cornea and lens block it from entering the eye. But people with aphakia have no lens, letting in some of the ultraviolet spectrum. They perceive it as a whitish blue or whitish violet. This is probably because all three cones have roughly the same sensitivity to ultraviolet light with the S-cones being just a bit more sensitive than the other two to it.

6. Rod cells are more light-sensitive than cone cells…

In fact, they’re about 100 times more sensitive. They’re so sensitive that they can detect a single photon. As a result, they are mostly responsible for your night vision. This is the reason why colors are harder to see when it’s dark.

Since there aren’t any rods in the center of your field of view, you can actually see dimmer things with your peripheral vision. One trick astronomers use to locate dim objects in the night sky is to look slightly away from the object they’re trying to see so that they can take advantage of their peripheral vision’s better light sensitivity.

7. …but they’re also much slower at processing information.

This makes rods worse at perceiving transient events, such as rapidly changing images or effects, than cones. For example, you can sort of see this difference by using your peripheral vision to look at a CRT monitor if you have one (yeah, I know it’s ancient technology). You can see it flicker much more with your peripheral vision than when you’re staring straight at it.

8. Your eyes are constantly moving.

Even when staring at a stationary object while staying still, your eyes are still moving around. It’s an involuntary reflex that continually stimulates your rods and cones by making them see new input. If they don’t continually receive new input, they just stop activating and your vision will go dark. You can see this effect by focusing your eyes on a small object in a very dimly-lit room without blinking. After a few seconds, your vision will start to darken.

9. There are six strong muscles responsible for each eye’s motion.

They are the lateral rectus, medial rectus, inferior rectus, superior rectus, interior oblique, and the superior oblique. They’re rather strong muscles for the amount of work they have to do, too (they’re just rotating an eyeball). But that strength is likely necessary for all the fast and precise movements the eyes have to do. For example, the brain can only process an image if it’s moving less than a few degrees per second across the retina. To ensure that you can still see something while you and/or the target is moving, those muscles have to keep pointing your eyes toward the object precisely.

10. Eye color can sometimes change.

Infants of European descent are often born with light-colored eyes. But as they age, certain cells in their irises start producing melanin (the same pigment that colors hair). This can darken their irises depending on how much is produced. Usually, an infant will obtain his or her final eye color by 3-6 months of age. But in some cases, the melanin levels can fluctuate depending on chemical reactions or hormonal changes in the body later on in life, leading to lighter or darker irises.

11. 20/20 vision doesn’t necessarily mean perfect vision.

What it means is you can see things at 20 ft. that an average person can see at 20 ft. It’s 6/6 in countries that use meters instead of feet. In other words, your vision is what it should be. But there are some people who have even sharper vision. For example, someone with 20/15 vision can see things at 20 ft. that an average person could only make out at 15 ft. In addition, it doesn’t take into account night vision, peripheral vision, or color vision.

12. Wearing corrective lenses won’t make your vision worse or dependent on them…

If you have bad vision, your eyes won’t get better on their own. Glasses and other corrective lenses are one of the few ways to fix that problem. They don’t hurt your vision. They merely focus the light coming into your eyes a certain way so that when it reaches the retina, it will be in perfect focus.

Wearing corrective lenses with the wrong prescription won’t damage your eyes either. They’ll probably strain your eyes and maybe give you a headache. But those are just temporary and will go away when you take them off.

13. …and not wearing corrective lenses if you need them won’t make your eyes deteriorate faster either.

It has pretty much the same effect as having glasses with the wrong prescription. Everything will be out of focus, and you’ll just have a hard time seeing anything. But you eyes won’t get any more damaged by not wearing them.

14. Reading in dim light won’t damage your vision.

The quality of lighting has no damaging effect on your vision. But it will make it harder for you to read since your central vision is dominated by cones, which don’t work well in low light. So, the worst it can do is strain your eyes or give you a headache.

15. Staring at an electronic screen from a close distance or for a long time won’t damage your eyes.

There’s nothing inherent about looking at an electronic screen that will hurt your vision. Like with the dim lighting situation, you might strain your eyes if your screen isn’t bright enough. Also when you’re looking at a screen you’re likely not blinking as much, which can dry your eyes out. This leads to them feeling irritated or sore.

As for viewing screens up close, it may actually be a sign of near-sightedness if that’s the distance you feel is most comfortable for viewing. The same goes for other viewable media, such as books. Otherwise, the most it can do is strain your eyes or give you a headache for all the same reasons above.

Eye doctors suggest the 20/20/20 rule for people who work long hours in front of a screen. Every 20 minutes take a 20 second break and look at something 20 ft. away. That should reduce eye strain.

16. Eating carrots won’t improve your vision.

Carrots contain a lot of vitamin A in the form of beta carotene. Your eyes need vitamin A to function so getting enough will help maintain your vision. But you can’t make your eyes any better than they are by eating more of it unless you currently have a vitamin A deficiency.

17. You don’t need blue-light filters to “protect” your eyes.

Ever since a study came out from the University of Toledo in 2018 that tested the effects of blue light on various cells with retinal (a photosensitive molecule essential for vision), scary headlines appeared in news outlets saying that blue light will make you go blind. And because a significant amount of the light electronic devices emit is blue, many companies started selling expensive blue-light filters for eyeglasses. They’ve been advertised to do everything from protecting your retina from “sharp” blue rays, preventing macular degeneration (damage to the macula, a part of the retina, caused by lack of antioxidants), and even helping prevent cancer.

Only problem is that study was more about how blue light can cause damage to cells with retinal, not whether it can cause damage in an actual eye. The study involved shining high-intensity blue light on various isolated cells in a dish (some of which don’t have retinal naturally), which doesn’t happen in a real eyeball. Another study involved rats, not humans. No study ever investigated the effects of blue light from screens on human eyes.

There are a number of reasons why the blue light from screens is nothing to worry about. First, screens are very dim compared to other sources of light. In fact, most of the blue light you see during the day is from the sky, not electronic screens. Roughly 30 times more, in fact. Did humans somehow evolve so that they’d go blind from simply being outside? Not a chance. Also, there’s no evidence that blue light from screens causes macular degeneration or any other damage to the retina. Lastly, the author of the 2018 study even confirmed that his study was not about damage to the eye and that blue light from screens won’t make you go blind at all.

The only thing blue light can do is mess with your melatonin levels, making you have trouble falling asleep. To stop that from happening, simply stop using your devices about an hour or two before bedtime or use your device’s night mode.

18. You can’t improve your vision through “vision training”.

There are a bunch of apps that claim to solve your vision problems with some “vision training” exercises (ex. Bates exercises). But there’s no evidence that they have any effect on your vision. Your vision tends to get worse as you age. This is mainly because of the lens slowly growing bigger over time or because of deterioration of various parts of the eye. The best you can do to fix bad vision it is to get corrective lenses or some other treatment, such as refractive surgery.

There is only one exception to this. For those who are suffering from convergence insufficiency (an inability to point both eyes exactly at a target), eye focusing exercises can help the eyes readjust properly. Even so, it’s much less effective for adults because their vision systems are pretty rigid by then.

19. Red light helps preserve your night vision.

Rods, which your night vision mostly depends on, aren’t sensitive to red light. So by using red light, the rods don’t lose their stores of photoreceptor protein (rhodopsin in the case of rods) and retinal (the two need to be “recharged” after activating in response to light). That leaves them still able to detect other light. That’s why astronomers and other people who need light to see something but don’t want to ruin their night vision use red light

20. It’s possible in certain circumstances to see “forbidden colors”.

You know purple to be an equal combination of red and blue. But what about red-green and yellow-blue? Can’t imagine what they look like? You technically can’t because the neurons that react to red or yellow turn off when they see green or blue, respectively, In other words, red exactly cancels out green and yellow exactly cancels out blue. As a result, you can’t see red and green (and yellow and blue) coming from the exact same place simultaneously. It’s the same with darkness and light. That’s why they’re “forbidden colors”.

But an experiment in 1983 tried to see them by using pictures with equally bright stripes of the two colors right next to each other and retinal trackers to make sure the image stayed in exactly the same place on the retina. This way, the cells on the retina always saw the same color. Apparently some of the participants of the experiment saw a vivid, new color that they couldn’t identify when the colors started blending in. So using a complex setup like this might let you see those mysterious colors.