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
It holds about 6.5 millilitres and grows from 16 to 17 mm at birth to full size by around age 12.
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 pupil of a human eye can range in diameter from about 2 mm to more than what?
8 mm
The iris does the adjusting, controlling how much light gets in.
Q 04How many muscles move each human eye?
Six
Four straight recti muscles and two obliques let the eye 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 06The fovea is less sensitive in dim light because it lacks what?
Rods
It covers only about 2 degrees of your visual field, which is why you cannot read out of the corner of your eye.
Q 07The retina is unusual among body tissues because it is technically part of what?
The central nervous system
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.
Q 08The natural blind spot in each eye is caused by what?
The optic nerve's exit point
It measures roughly 7.5 degrees high by 5.5 wide, and the brain fills it in so you never notice.
Q 09The jelly-like substance filling the back of the eyeball is called what?
Vitreous humour
It is mostly water and proteins; the watery aqueous humour sits in the front chambers between the cornea and the lens.
Q 10How long does initial dark adaptation take when you walk into a dark room?
About four seconds
The full process is about 80 percent complete after thirty minutes.
Q 11Roughly what percentage of people worldwide have brown eyes?
79%
Blue is next at 8 to 10 percent, then hazel at about 5 percent of the world and 18 percent of Americans.
Q 12Which is the rarest human eye colour, seen in about 2 percent of people?
Green
Grey comes second at about 3 percent, and heterochromia, two different coloured eyes, affects under 1 percent.
Q 13Blue eyes contain no blue pigment. What actually makes them look blue?
Light scattering in the stroma
It is Tyndall scattering, a cousin of the Rayleigh scattering that makes the sky blue; the same trick produces other light eye colours.
Q 21Colour blindness is sometimes called 'daltonism' after which English chemist?
John Dalton
He left instructions for his eyes to be examined; DNA analysis of a preserved eyeball in 1995 confirmed he had deuteranopia.
Q 22The dotted-circle plates used to screen for red-green colour deficiency are named after whom?
Ishihara
There is still no approved cure, and colour blindness bars people from flying aircraft or driving trains in many countries.
Q 23In myopia, or short-sightedness, where does light from distant objects come to a focus?
In front of the retina
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.
Q 14Research suggests all blue-eyed people share a single ancestor whose HERC2 mutation arose roughly when?
6,000 to 10,000 years ago
The mutation dialled down the OCA2 gene and with it melanin production; it probably arose near the Black Sea during the Neolithic.
Q 15The yellowish pigment that gives amber and hazel eyes their colour is called what?
Lipochrome
Mammals are born with blue eyes, and any darkening happens after birth as melanin builds up.
Q 16In a laboratory setting, adults blink on average about how many times per minute?
17
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.
Q 17Newborn babies blink roughly how often?
Once or twice a minute
The rate climbs through childhood and reaches adult levels in adolescence.
Q 18The tears you shed while chopping onions belong to which of the three types?
Reflex
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.
Q 19Which antibacterial enzyme in tears dissolves the outer coating of certain bacteria?
Lysozyme
Alexander Fleming discovered it in 1922, six years before penicillin, after a drop from his nose fell onto a bacterial culture.
Q 20Red-green colour blindness affects roughly what share of men?
1 in 12
Only about 1 in 200 women are affected, because the genes sit on the X chromosome and women have a spare copy.
Q 24A cataract is a clouding of which part of the eye?
The lens
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.
Q 25Eduard Zirm performed the first successful corneal transplant in 1905 in a town in what is now which country?
Czechia
Olomouc was then in Moravia; corneal grafts remain among the most common transplants, with about 100,000 a year.
Q 26The letter chart used to test eyesight was devised in 1862 by which Dutch ophthalmologist?
Herman Snellen
It has eleven lines, and the classic version 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
Probably in Pisa or Florence; 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
He suffered from both short-sightedness and presbyopia, and letters suggest he made them decades earlier than once thought.
Q 30In which city was Ibn al-Haytham, the 'father of modern optics', born?
Basra
Ibn al-Haytham's Book of Optics also gave the first clear account of the camera obscura and influenced Kepler and Galileo.