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.