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This eye trivia quiz has 50 free questions with answers and covers the human eye and vision from the cornea to the visual cortex: how big an eyeball is, what the iris and lens do, rods and cones, the blind spot, why blue eyes have no blue pigment, how rare green eyes are, why babies blink so little, the three kinds of tears, colour blindness and who first described it, myopia's rise in East Asia, cataracts and the first artificial lens. Then it goes wider: the mantis shrimp's 12 to 16 photoreceptor types, the colossal squid's dinner-plate eyes, owls that cannot move their eyes, goats' horizontal pupils, scallops with 200 mirror eyes and jellyfish with 24 eyes and no brain. History rounds it out with Ibn al-Haytham, the first spectacles in Italy, Snellen's chart, the first corneal transplant, contact lenses and LASIK. It is written for science classes, optometry students, pub quiz rounds and the curious. Easy questions cover the parts of the eye everyone learns at school; hard ones ask about photoreceptor counts and who patented laser eye surgery. Every answer has been checked against a reliable source and each question shows its citation once you have answered.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Roughly how wide is a typical adult human eyeball from front to back?
24 mm
A typical adult eyeball measures about 24 mm from front to back and holds roughly 6.5 millilitres; at birth it is only about 70 percent of that length.
Q 02Which part of the eye provides most of its focusing power?
The cornea
The clear dome is about 11.5 mm across and half a millimetre thick at the centre; the lens does the fine-tuning.
Q 03The human pupil's diameter ranges from about 2 mm up to roughly what in darkness?
8 mm
In darkness the pupil opens to about 8 mm across, while in bright light it shrinks to roughly 2 mm; the muscular iris makes the adjustment.
Q 04How many muscles move each human eye?
Six
Six extraocular muscles move each eye: four straight recti and two obliques, which together let it look in every direction and roll slightly.
Q 05Which photoreceptor cells in the retina work in dim light and see only in shades of grey?
Rods
The retina holds around 7 million cones and somewhere between 75 and 150 million rods.
Q 06Which part of the retina, packed with cone cells, delivers the sharpest central vision?
Fovea
Fovea is the tiny central pit of the retina where cone cells are densest; it covers only the central two degrees of the visual field, which is why reading needs a direct gaze.
Q 07The retina is unusual among body tissues because it is technically part of what?
The central nervous system
The central nervous system includes the retina, which grows out of the embryonic brain as an outgrowth of the diencephalon; its blood vessels are the only ones a doctor can inspect directly without surgery.
Q 08The natural blind spot in each eye is caused by what?
The optic nerve's exit point
The optic nerve's exit point, the optic disc, has no photoreceptors, leaving a gap roughly 7.5 degrees high by 5.5 degrees wide that the brain fills in unnoticed.
Q 09The jelly-like substance filling the back of the eyeball is called what?
Vitreous humour
Vitreous humour is a clear gel, about 99 percent water with fine collagen fibres, that fills the large rear chamber and holds the eyeball's shape; the watery aqueous humour fills the front chambers.
Q 10Which thin membrane lines the eyelids' inside and covers the white of the eye?
Conjunctiva
Conjunctiva is the thin, clear mucous membrane lining the inner eyelids and covering the sclera; its inflammation, conjunctivitis, is the familiar pink eye.
Q 11A shortage of which vitamin is the classic cause of night blindness?
Vitamin A
Vitamin A supplies retinal, the light-catching half of rhodopsin, the pigment behind dim-light vision; when it runs short the eyes struggle in the dark, a condition called nyctalopia.
Q 12What is the age-related loss of near focusing, common after 40, called?
Presbyopia
Presbyopia develops as the eye's focusing element stiffens with age and can no longer adjust for close work, which is why reading glasses become common after about 40.
Q 13What gives blue eyes their colour, since the iris holds no blue pigment?
Light scattering in the stroma
Q 21Which English chemist, colour-blind himself, lends his name to a term for the condition?
John Dalton
John Dalton described his own colour blindness in 1798 and left instructions for his eyes to be examined after death; DNA from a preserved eyeball confirmed in 1995 that he had deuteranopia.
Q 22The dotted-circle plates used to screen for red-green colour deficiency are named after whom?
Ishihara
Ishihara plates were devised by the Japanese ophthalmologist Shinobu Ishihara in 1917; each shows a number or path in coloured dots that people with red-green deficiency read differently or cannot see.
Q 23In myopia, or short-sightedness, where does light from distant objects come to a focus?
In front of the retina
Light scattering in the stroma, the iris's front layer of collagen fibres, sends short blue wavelengths back out; it is the Tyndall effect, a cousin of the Rayleigh scattering that makes the sky blue.
Q 14Research suggests all blue-eyed people share a single ancestor whose HERC2 mutation arose roughly when?
6,000 to 10,000 years ago
Copenhagen researchers dated the change to 6,000 to 10,000 years ago; the HERC2 mutation dialled down the neighbouring OCA2 gene and with it the melanin that makes irises brown.
Q 15The yellowish pigment that gives amber eyes their colour is called what?
Lipochrome
Lipochrome is the yellowish pigment deposited in the iris that gives amber eyes their golden, coppery colour; the shade is uncommon in humans but familiar in wolves and many domestic cats.
Q 16In a laboratory setting, adults blink on average about how many times per minute?
17
Resting adults averaged 17 blinks a minute in the classic study of spontaneous blinking, each lasting about 100 to 150 milliseconds; the rate falls to four or five while reading, which is why screens dry eyes out.
Q 17Newborn babies blink roughly how often?
Once or twice a minute
Once or twice a minute is typical for a newborn, against well over a dozen blinks for a resting adult; the rate climbs steadily through childhood.
Q 18The tears you shed while chopping onions belong to which of the three types?
Reflex
Reflex tears flood the eye in response to irritants such as onion vapour, dust or bright light; basal tears keep the surface wet all day and emotional tears are the third kind.
Q 19Which antibacterial enzyme in tears attacks the cell walls of certain bacteria?
Lysozyme
Lysozyme breaks down the peptidoglycan cell walls of many bacteria; Alexander Fleming discovered it in 1922, well before his penicillin of 1928, after a drop from his nose fell onto a culture plate.
Q 20Red-green colour blindness affects roughly what share of men?
1 in 12
About 1 in 12 men have red-green colour deficiency, against only about 1 in 200 women; the responsible genes sit on the X chromosome and women carry a second copy.
In front of the retina is where a myopic eye focuses distant light, usually because the eyeball has grown slightly too long from front to back; outdoor time in childhood appears to protect against it.
Q 24A cataract is a clouding of which part of the eye?
The lens
The lens sits behind the iris and clouds with age; cataract surgery swaps the cloudy lens for a plastic implant, a technique Harold Ridley pioneered in London in 1949.
Q 25Eduard Zirm performed the first successful corneal transplant in 1905 in a town in what is now which country?
Czechia
Czechia is home to Olomouc, the Moravian town, then in Austria-Hungary, where Zirm performed the graft on a labourer blinded by lime burns in December 1905.
Q 26The letter chart used to test eyesight was devised in 1862 by which Dutch ophthalmologist?
Herman Snellen
Herman Snellen published his chart in 1862 while working under Franciscus Donders in Utrecht; the classic version has eleven lines and uses only the letters C, D, E, F, L, O, P, T and Z.
Q 27A standard eye test is read from how far away?
20 feet
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.
Q 28The first eyeglasses were made in about 1290 in which country?
Italy
Italy is the accepted birthplace, probably Pisa, where the earliest spectacles appeared between about 1286 and 1290; the story that a Florentine named Salvino D'Armate invented them has been debunked.
Q 29Which American statesman is credited with inventing bifocal spectacles?
Benjamin Franklin
Benjamin Franklin, who needed one pair of glasses for distance and another for reading, described his 'double spectacles' in a letter of 1785, with the distance half above and the reading half below.
Q 30In which city was Ibn al-Haytham, the 'father of modern optics', born?
Basra
Basra, in what is now southern Iraq, was his birthplace around 965; he later worked in Cairo, where his Book of Optics gave the first clear account of the camera obscura and influenced Kepler.