12 Unbelievable Facts About Human Eyes That You Didn’t Know!

Even as you read this, your eyes are working at an incredible pace and doing it silently and stealthily. Ask yourself though – how much do you know about this incredible organ? For instance, did you know that every time you light up a cigarette, you are reducing your night vision? Or that an average blink lasts 300-400 milliseconds?

Here are some other facts about human eyes, so that you can appreciate the superhuman strength of the deceptively simple human eye.

1. The focusing muscle of the human eye moves about 1000,000 times every day.

Eye muscles
Image source: giphy

It’s crazy how much the focusing muscle of the eye gets done. On an average, this spectacular muscle of the human eye moves about 1000,000 times every single day. If that doesn’t blow your mind, let’s put it in perspective. For your leg muscles to get the same kind of workout, you would have to walk 50 miles, every day.(source)

2. Until about 10,000 years ago, all human beings had brown eyes. Blue eyes are a more recent phenomenon caused by a genetic mutation, a sign that humans are still evolving. 

Blue eyes
Image source

Originally, every single human being had brown eyes. However, about 10,000 years ago, someone who lived near the Black Sea developed a genetic mutation that turned brown eyes to blue.  There are many theories about how blue eyes have still persisted, so many generations later. For instance, one theory suggests that blue eyes act as sort of a paternity test. This means that since there is strong evolutionary pressure on a man not to invest his paternal resources in another man’s child, he would actively seek out blue-eyed mates. Since it is impossible for two blue-eyed partners to create a brown-eyed baby, this instinct may have led to the genetic preservation of blue eyes.(source)

3. How far can the human eye see? Assuming a flat earth and complete darkness, the human eye is sensitive enough to spot a candle flame, flickering up to 30miles (48 km) away.

How far eye can see
Image source

Although the Earth’s surface curves out of sight at a distance of 3.1 miles (5 km), our ability to see extends far beyond that distance. If Earth were completely flat, or if you were standing on top of a mountain surveying a larger-than-usual patch of the planet, you could see bright lights that are hundreds of miles away! On a particularly dark night, you could even see a candle flame flickering in the distance, specifically up to 30 miles away.

The eye’s ability to perceive things in the distance depends on how many particles of light, or photons, an object emits. After extensive research, vision scientist Selig Hecht discovered that the absorption of 5 to 14 photons, or, equivalently, the activation of just 5 to 14 rod cells is enough to tell your brain that you’re seeing something. The farthest object that is visible to the naked eye is the Andromeda galaxy, wherein its constituent 1 trillion stars collectively emit enough light for a few thousand photons to hit each square centimeter of Earth every second.(source)

4. Both our left and right eyes have independent blind spots.  

The human eye has a blind spot on the retina, at the point where the optic brain leads back to the brain. However, the wonderful thing is that the left and right eyes have blind spots that are aligned symmetrically. What this means is that one eye’s field of vision will compensate for the other’s loss of vision, most of the time.

Are you now wondering if you can locate the blind spots in both your eyes? Yes, you can. There’s a rather simple test that can help you identify the blind spot in each of your eyes. In order to find the blind spot on the right eye – shut your left eye, focus the right eye on a single point and see if anything vanishes from your line of sight, about 20 degrees to the right. That’s your blind spot. Repeat the same steps for your left eye.(source)

In order to find the blind spot of the right eye, it is necessary to close the left eye, focus the right eye on a single point, and see if anything vanishes from vision some 20 degrees right of this point. The following diagram has a set of characters on the left hand side, and black circle on the right. Keeping your head motionless, with the right eye about 3 or 4 times as far from the page as the length of the red line, look at each character in turn, until the black circle vanishes.

right eye blind spot test

 

5. The human eye evolved from a plain spot of photoreceptive cells.

eye evolution
Image source: wikipedia

In fact, eyes went through a few stages of evolution, from photoreceptor proteins to the eye as we know it, today. The earliest predecessors of the eye were photoreceptor proteins, found in unicellular organisms, called ‘eyespots’. Eyespots can only sense brightness. Then, it became important for organisms to be able to discriminate the direction that light was coming from. This was made possible by the multicellular eyepatch, gradually depressed into a cup, which first granted the ability to discriminate brightness in directions, then in finer and finer directions as the pit deepened.

The development of the retina came next. When a photon is absorbed by the chromophore(part of a molecule responsible for its colour), a chemical reaction causes the photon’s energy to be transduced into electrical energy and relayed, in higher animals, to the nervous system. These photoreceptor cells form part of the retina, a thin layer of cells that relays visual information to the brain.

Then, things happened very quickly. The next stage was the discovery that reducing the width of the light opening became more efficient at increased visual resolution, than simply deepening the cup. These kind of eyes still lacked a cornea or lens which, sure enough, came to be in the next stage of eye evolution.(source)

6. The human eye, an incredible organ, can distinguish about 10 million different colors.

one million colors
Image source: wikipedia

Essentially, there are three primary colours – red, green and blue – that make up the millions of colors distinguishable to the human eye. Each eye contains three receptors, one for each primary colour, that are responsible for the creation of the experience of color, when stimulated in various combinations. This is known as the Young-Helmholtz Trichromatic Theory. Thus, it follows that those who have defective cones cannot see certain colours and are known to be color-deficient.(source)

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