40 Things You Didn’t Know About Your Eyes

From blind spots in our retinas to an eye
colour that didn’t exist until recently, we count 40 bizarre facts about our own eyeballs
40 Human eyes blink an average of 17 times a
minute. This equates to 14,280 times a day or 5.2 million times a year. 39
Blinking removes debris from the eye’s surface by spreading tears over it. The tears help
moisten and lubricate the eyes. They also have anti-bacterial properties. 38
Eyes heal quickly. With proper care, corneal abrasions can repair within 48 hours. 37
Newborns don’t produce tears. While they do make crying sounds, their tears don’t
start flowing until they are 4–13 weeks old. 36
Approximately six in a thousand people are born with heterochromia iridus. This is a
condition where the person’s eye are two different colours. 35
A shark’s corneas are very similar to those of humans’. Because of this similarity,
shark corneas have been used as replacements in human eye surgeries. 34
Schizophrenia can be diagnosed with 98.3% accuracy using a simple eye test that tracks
eye movement abnormalities. 33
Dogs are the only species other than humans that seek visual cues from other individuals’
eyes. Dogs only do this when interacting with humans. 32
‘Impossible’ or ‘forbidden’ colours are colours that are too complex for the human
eye. Although it is impossible to perceive them under regular viewing conditions, they
can be seen in special circumstances 31
The pupil of your eye expands as much as 45% when you look at someone you love. Similarly,
a person’s pupils may dilate when looking at someone they are sexually attracted to. 30
It’s possible for your eyes to be sunburned. Prolonged sun exposure eventually leads to
a thickening of the eye tissue, which might require eye surgery. 29
Your retinas perceive the outside world as upside-down. The brain must then flip and
make sense of the image. 28
Eye colour – or our perception of it – can change with lighting conditions. Neither blue
nor green pigments are present in the human iris or ocular fluid. 27
Octopus eyes have no blind spot and evolved separately from vertebrate eyes. This is an
example of convergent evolution. 26
Up until 10,000 years ago, brown was the only known human eye colour. This changed when
a person living by the Black Sea developed a genetic mutation that made their eyes blue. 25
If one red eye appears in a flash photograph, there is a chance this person has a strand
of treatable eye cancer called Leukocoria. For this test to work, both eyes must be looking
directly at the camera. 24
Human retinas cannot detect the colour red. Our ‘red’ receptor only detects colours
in the yellow–green spectrum; thus, our brains must combine multiple signals to perceive
red. 23
It’s possible for eyesight to improve with age. However, this can be a sign that something
is wrong with a person’s overall health. 22
Your eye’s lens sits behind the iris and is roughly the size of an M&M candy. Developing
a cataract in your eye is like developing a peanut in that M&M. 21
Our peripheral vision has a very low-resolution, and is almost black-and-white. But we don’t
notice this because our eyes move and fill in the details. 20
The visual centres in the brain are located at the lower back part of your head. This
is why people with head injuries can sometimes experience temporary blindness. 19
Tetrachromacy is a rare genetic mutation occurring in 2% of women. It gives them an extra retinal
cone, allowing them to see 100 million colours. 18
Staring directly into the sun will burn a spot in the retina, causing permanent blindness.
This is called solar retinopathy. 17
Iris pigmentation develops over the first year of life. This means our eyes are darker
now than they were when we were newborns. 16
Our eyeballs grow just like the rest of our body. At birth, they are roughly 16 centimetres
wide; by puberty, they will have grown to a maximum width of 24 millimetres. 15
‘20/20 vision’ isn’t perfect vision. It simply means a person can see 20 feet in
front of them with the same clarity as a normal-visioned person. 14
If you’re short-sighted, your eyeball is longer than normal. If you’re farsighted,
it’s shorter than average. 13
The composition of tears differs depending on whether you’re crying, yawning or have
an irritant in your eye. 12
In order for you to see, your brain must interpret the signals it receives from your eyes. Optical
illusions occur when there are discrepancies between what your brain and eyes perceive. 11
Our eyes constantly make tiny involuntary jerking movements called ‘microsaccades’.
These stop objects from fading from our vision. 10
Although the human eye is thought to be capable of detecting around 10 million unique colours,
they only have the capacity to detect 30 shades of grey. 9
The refractive power of a human eye lens is approximately 18 dioptres, roughly one third
of the eye’s total power. This makes an eye’s lens quicker than a camera’s. 8
Human eyes contain 107 million cells – all of which are light sensitive. Seven million
cones help with the detection of colour and detail, while 100 million rods allow us to
distinguish black and white. This means that less than a tenth of our visual receptors
detect colour. 7
Human eyes have a small blind spot in the back of the retina where the optic nerve attaches.
We don’t notice this hole in our vision because each eye works together to fill the
other’s blind spot. 6
The human eye can only make smooth (non-saccadic) motions if it’s tracking a moving object.
A person cannot will their eye into making a smooth motion. 5
Pirates used eye patches to quickly adjust their eyes from above to below deck. They
would have one eye trained for the bright light and the other for dim, below deck lighting. 4
All kittens are born with blue eyes, which – if they are going to do so – will change
colour by around 8 weeks of age. 65–85% of all white cats born with blue eyes are
deaf. 3
To detect nocturnal predators, many animal species will sleep with one eye open. One
hemisphere of their brain is asleep while the other is awake. 2
Our two eyes give us depth perception. Comparing two images allows us to determine how far
away an object is from us. 1
People generally read 25% slower from a computer screen compared to paper. Reports also suggest
that late-night screen-reading may be damaging to our eyes.

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