50 Fun Facts About Eyes

Roughly how wide is a typical adult human eyeball from front to back?

It holds about 6.5 millilitres and grows from 16 to 17 mm at birth to full size by around age 12.

Which part of the eye provides most of its focusing power?

The clear dome is about 11.5 mm across and half a millimetre thick at the centre; the lens does the fine-tuning.

The pupil of a human eye can range in diameter from about 2 mm to more than what?

The iris does the adjusting, controlling how much light gets in.

How many muscles move each human eye?

Four straight recti muscles and two obliques let the eye look in every direction and roll slightly.

Which photoreceptor cells in the retina work in dim light and see only in shades of grey?

The retina holds around 7 million cones and somewhere between 75 and 150 million rods.

The fovea is less sensitive in dim light because it lacks what?

It covers only about 2 degrees of your visual field, which is why you cannot read out of the corner of your eye.

The retina is unusual among body tissues because it is technically part of what?

It grows out of the developing brain, and its blood vessels are the only part of the nervous system a doctor can look at directly without surgery.

The natural blind spot in each eye is caused by what?

It measures roughly 7.5 degrees high by 5.5 wide, and the brain fills it in so you never notice.

The jelly-like substance filling the back of the eyeball is called what?

It is mostly water and proteins; the watery aqueous humour sits in the front chambers between the cornea and the lens.

How long does initial dark adaptation take when you walk into a dark room?

The full process is about 80 percent complete after thirty minutes.

Roughly what percentage of people worldwide have brown eyes?

Blue is next at 8 to 10 percent, then hazel at about 5 percent of the world and 18 percent of Americans.

Which is the rarest human eye colour, seen in about 2 percent of people?

Grey comes second at about 3 percent, and heterochromia, two different coloured eyes, affects under 1 percent.

Blue eyes contain no blue pigment. What actually makes them look blue?

It is Tyndall scattering, a cousin of the Rayleigh scattering that makes the sky blue; the same trick produces other light eye colours.

Research suggests all blue-eyed people share a single ancestor whose HERC2 mutation arose roughly when?

The mutation dialled down the OCA2 gene and with it melanin production; it probably arose near the Black Sea during the Neolithic.

The yellowish pigment that gives amber and hazel eyes their colour is called what?

Mammals are born with blue eyes, and any darkening happens after birth as melanin builds up.

In a laboratory setting, adults blink on average about how many times per minute?

Each blink lasts 100 to 150 milliseconds, and the rate drops to four or five a minute when you are reading, which is why screens dry your eyes out.

Newborn babies blink roughly how often?

The rate climbs through childhood and reaches adult levels in adolescence.

The tears you shed while chopping onions belong to which of the three types?

Basal tears keep the cornea wet all day, and emotional tears are the third kind; all three drain into the nose, which is why crying makes it run.

Which antibacterial enzyme in tears dissolves the outer coating of certain bacteria?

Alexander Fleming discovered it in 1922, six years before penicillin, after a drop from his nose fell onto a bacterial culture.

Red-green colour blindness affects roughly what share of men?

Only about 1 in 200 women are affected, because the genes sit on the X chromosome and women have a spare copy.

Colour blindness is sometimes called 'daltonism' after which English chemist?

He left instructions for his eyes to be examined; DNA analysis of a preserved eyeball in 1995 confirmed he had deuteranopia.

The dotted-circle plates used to screen for red-green colour deficiency are named after whom?

There is still no approved cure, and colour blindness bars people from flying aircraft or driving trains in many countries.

In myopia, or short-sightedness, where does light from distant objects come to a focus?

Usually the eyeball has simply grown too long; up to 93 percent of people in South Korea are affected, and outdoor time in childhood seems to protect against it.

A cataract is a clouding of which part of the eye?

It is the leading cause of blindness worldwide, and surgery swaps the cloudy lens for a plastic one, a technique Harold Ridley pioneered in 1949.

Eduard Zirm performed the first successful corneal transplant in 1905 in a town in what is now which country?

Olomouc was then in Moravia; corneal grafts remain among the most common transplants, with about 100,000 a year.

The letter chart used to test eyesight was devised in 1862 by which Dutch ophthalmologist?

It has eleven lines, and the classic version uses only the letters C, D, E, F, L, O, P, T and Z.

A standard eye test is read from how far away?

That is where 20/20 comes from: seeing at 20 feet what a person with normal acuity sees at 20 feet. Reading only the big top letter is 20/200, legally blind in the US.

The first eyeglasses were made in about 1290 in which country?

Probably in Pisa or Florence; the story that a Florentine named Salvino D'Armate invented them has been debunked.

Which American statesman is credited with inventing bifocal spectacles?

He suffered from both short-sightedness and presbyopia, and letters suggest he made them decades earlier than once thought.

In which city was Ibn al-Haytham, the 'father of modern optics', born?

Ibn al-Haytham's Book of Optics also gave the first clear account of the camera obscura and influenced Kepler and Galileo.

Which German ophthalmologist made the first successful contact lens, of blown glass, in 1888?

Leonardo da Vinci had sketched the idea in 1508; soft hydrogel lenses came from Czech chemists Wichterle and Lím in 1959.

In LASIK surgery, what does the excimer laser reshape?

Gholam Peyman patented the idea in 1989 and the FDA approved the first laser for it in 1998; more than 40 million procedures were done worldwide by 2016.

The mantis shrimp's eyes contain how many types of photoreceptor cell, compared with four in humans?

Six species can also detect circularly polarised light, which no other animal is known to do.

Which animal has the largest eyes ever known, up to about 40 cm across?

A specimen examined in 2008 had an eye 27 cm across even after it had partly collapsed.

Owls have to swivel their heads up to 270 degrees because their eyes are what?

They manage the twist with 14 neck vertebrae, twice as many as humans, and arteries with extra room so the blood keeps flowing.

A chameleon can watch two different objects at once because it can do what?

When it hunts, both eyes swing forward for stereo vision and the tongue reaches the prey in as little as 0.07 seconds.

What shape are a goat's pupils?

The shape gives grazing animals a wide view to either side while shielding them from glare overhead.

Scallops' 200 tiny eyes focus light using what, instead of a lens?

The mirror is made of guanine crystals, the same idea as a reflecting telescope.

How many eyes does a box jellyfish have, despite having no brain?

Six sit on each of four sensory clubs, and some have proper corneas, lenses and retinas.

The reflective layer that makes a cat's eyes shine in headlights, which humans lack, is called what?

It bounces light back through the retina for a second pass; in cats it is built from riboflavin and zinc.

The red-eye effect in flash photographs is mainly caused by light reflecting off which blood-rich layer?

The choroid sits behind the retina and feeds it; the flash goes in and bounces straight back out red.

The combined field of view of both human eyes spans roughly how many degrees horizontally?

About 120 degrees of that is seen by both eyes at once, which is what gives you depth perception.

Besides rods and cones, which retinal photoreceptors help set the circadian rhythm?

Identified in the early 2000s, they respond across the full range of light intensities and also help adjust how much light reaches the retina.

What is the job of the seventh extraocular muscle in each orbit, the levator palpebrae superioris?

The other six, four recti and two obliques, turn the globe in almost pure rotation, with only about a millimetre of sideways shift.

In which direction does a single human eye see furthest, about 107 degrees from the fixation point?

The brow limits upward vision to about 30 degrees and the nose cuts the nasal field to about 45, while the downward field reaches roughly 70.

Which part of the brain controls saccades, the quick simultaneous jumps both eyes make together?

After each saccade the iris re-adjusts the pupil to fix exposure; smaller movements include drift, ocular tremor and microsaccades.

During accommodation for near objects, what shape does the lens relax into?

The ciliary muscles narrow, slackening the suspensory fibres so the lens bulges and bends light more strongly onto the retina.

What is the white ring that develops around the edge of the cornea with age called?

Ageing also liquefies the vitreous gel, so the specks known as floaters gradually multiply.

Which two antioxidant pigments in the macula are linked to a lower risk of age-related macular degeneration?

They bind free radicals excited by high-energy light; the condition affects roughly 1.75 million Americans each year.

After entering total darkness, roughly how long does it take for dark adaptation to be 80 percent complete?

Any interruption by light restarts the whole process, which relies on the retina's rod photoreceptors.