a smart limits

[music] this episode is supported by the great coursesplus i’ve never seen a gorilla with glasses,a chameleon with contacts, or a moose with a monocle. although i did see a spectacledbear once. but like 7 out of 10 americans, my eyes don’tbend light correctly, so i’ve worn glasses since fifth grade.it’s a confusing question: how did our species survive the trials of natural selection ifso many of us can’t see? we can’t take otters or ostriches to theophthalmologist, so it’s hard to know exactly how common bad eyesight is in the animal kingdom.

but if you can smell, hear, or feel your way through life, good eyes are expendable. many species that rely on seeing sacrifice clear vision all over to see extra-sharp where natural selection demands. for most visual species, you’re more likelyto become a meal before you pass on your blurry genes, so they vanish from the population. but modern humans live a pretty comfortablelife, so natural selection has let us slip through with bad eyesight. the lens inside your eye isn’t rigid—morelike really firm jell-o. it’s made from flexible proteins called crystallin.

they let visible wavelengths pass through, but the lens’s rounded shape bends the light. refraction: just like what happens where air meets water. we can’t focus by moving our lens closeror farther from the retina, the way a camera would. we actually squish and pull our lensto change how it bends light. now, objects reflect light in all directions. the farther away an object is, the angles between the light rays entering our eyes are smaller,and our lens doesn’t need to bend them as much, so tiny muscles relax and flatten it. as an object moves closer, light rays are

hitting our lens at wider angles, so musclessqueeze the lens into a rounder shape to bring the focal point right onto the retina. we do all this involuntarily—healthy eyescan focus from infinity to here in less than half a second. but in many of us, this process doesn’twork perfectly. people with myopia, or nearsightedness, have trouble focusing on objects far away, often because the eye is too long—kind of football-shaped. the lens focuses the imagein front of the retina. or if you have astigmatism, like me, your cornea is too round, producingweird blurry effects.

this is corrected by using a lens that spreadsthe light out just a tiny bit before it enters the eye. the focal point now hits right on the retina. if the eye is too short or the lens can’tquite squish into a round enough shape, the focal point moves behind the retina when trying to view things up close. we call this hyperopia, or farsightedness. and it’s corrected usinga lens that slightly focuses the light in front of your eye—kind of like wearing tinymagnifying glasses. as we age, people often get presbyopia: ourlens becomes less elastic, unable to squish enough to bring the closest objects into focus.

if somebody already requires distance correction, often they’ll be be fitted with bifocalsor contacts with two different types of lenses: one for far and one for near. contact lenses correct these problems thesame way as glasses do—you just don’t notice the different lens shapes on the tip of your finger. corrective surgeries like lasik don’t actually work on your lens. they use lasersto reshape the round cornea in front of it, adjusting how it naturally bends light. strangely, over the past half century, eyesightis getting worse in developed countries.

60 years ago, nearsightedness affected 1 in 10chinese people, but today it affects 90% of the younger generation. in seoul, korea alone,97% of 19-year-old males need corrective lenses. the change is too fast to be due to just genetics.it was originally thought that the culprit was too much time spent reading or staringat devices up close, but newer research suggests that the biggest risk factor could be kidsspending less time outside under bright sunlight. so how good can our vision get? could we seelike eagles? when our lenses—natural or prosthetic—are just right, the limit of human visionis lower than 20/20—actually closer to 20/8. professional athletes in visual reflex sportslike baseball are often down in this range.

the limit to our vision doesn’t come frombeing able to bring things into focus; it’s because the cone cells on our retina are onlypacked so close together. like pixels on a camera sensor, we can’t pick up detail smaller than a single cell. birds of prey like hawks have more tightlypacked cells on their retinas, so they can see more detail than we can. what's strange is people who have had theirnatural lenses removed can see expanded wavelengths, down into the ultraviolet range. claude monet,the artist, was one of these people. we made a video about him; you should check it out. we might not ever see like superman, but thanksto physics, we can bring our vision into focus.

stay curious. thanks to the great courses plus for sponsoring this episode. the great courses plus is a service that allows you to learn about a range of topics from educators, including ivy league professorsand other schools from around the world. go to thegreatcoursesplus.com/ok and get access to a library of different video lectures about science, math, history, literature… or even how to cook, play chess,or become a photographer. new subjects, lectures, and professorsare added every month. let’s say you wanna know what happens toyour vision after your eye—

they’ve got a course called understanding the brain. with the great courses plus, you can watch as many different lectures as you want—anytime, anywhere— without any tests or exams. help support my show and start your one-month trialby going to thegreatcoursesplus.com/ok. unlike most viruses, not only can zika cross the placenta from mother to child— but we now know it specifically targets developing brain cells. worse, zika also kills radial glial cells,

which normally act as a scaffold to shape the developing brain. there’s no treatment or vaccine, and symptoms are usually so mild that most people don’t even know they have it the world health organization has resortedto telling women in zika-affected areas, “don’t get pregnant”—which is a pretty good signwe don’t have any good options.