OAT Practice Questions (2026) — Free Sample Questions with Answers

Lenses That Respond to Light

¹Photochromic spectacle lenses darken in bright sunlight and clear again indoors, sparing the wearer from switching between ordinary glasses and sunglasses. The word photochromic describes any material whose color changes reversibly when it absorbs light. Although the effect seems simple, it depends on a balance between two competing processes, and understanding that balance explains both the strengths and the familiar frustrations of these lenses.

²The first commercially successful photochromic lenses, introduced in the 1960s, were made of glass containing a great many microscopic crystals of silver halide, the same light-sensitive compound used in photographic film. When ultraviolet light strikes a crystal, it frees electrons that convert some silver ions into tiny clusters of metallic silver. These clusters absorb visible light, and the lens darkens. In photographic film the change is made permanent by chemical processing. In a lens, however, the halogen atoms released by the light remain trapped in the surrounding glass, close to where they were formed. When the light is removed, they recombine with the silver, the clusters disappear, and the lens clears. Small amounts of copper added to the glass help this reverse reaction proceed.

³Most lenses sold today are plastic, and plastic cannot hold silver halide crystals in the same way. Instead, plastic photochromic lenses contain organic dye molecules dissolved in the lens material or in a thin layer near its surface. In their resting form, these molecules absorb only ultraviolet light and are therefore colorless. Absorbing an ultraviolet photon causes a ring of atoms within the molecule to break open, and the molecule rearranges into a flatter, more extended shape that absorbs visible light. The opened form is less stable than the closed one, and with time it snaps back to its original shape, restoring the clear state.

⁴In both kinds of lens, the darkening reaction is driven by light, while the reverse reaction, at least in the most common designs, is driven mainly by heat. At any moment, molecules or silver clusters are being switched on by ultraviolet light and switched off by thermal motion. Under steady conditions the two processes reach a balance, and the darkness of the lens reflects that balance rather than the brightness of the light alone.

⁵This explains why temperature matters so much. On a cold, bright winter day, the reverse reaction is slow, so the lens becomes very dark; on a hot summer beach, with equally strong sunlight, the faster reverse reaction keeps the lens noticeably lighter. The same principle governs how quickly lenses clear. A wearer who walks from bright snow into a warm building may wait several minutes for cold lenses to fade, whereas lenses that are already warm clear more quickly. Manufacturers try to choose dyes that darken strongly yet fade quickly, but the two goals pull in opposite directions: a molecule that returns readily to its clear form also tends to darken less in the sun.

⁶Another common complaint concerns driving. Most automobile windshields block nearly all of the ultraviolet light from the sun while transmitting most visible light. Because conventional photochromic lenses are activated by ultraviolet light, they darken only slightly inside a car, even on a bright day. Some newer designs use dyes that also respond to the shortest wavelengths of visible light, allowing partial darkening behind a windshield.

⁷Finally, photochromic dyes do not last indefinitely. Each switching cycle carries a small chance that a molecule will undergo an unwanted side reaction and permanently lose its ability to switch, a process called fatigue. Over years of use, plastic lenses may darken less fully or retain a slight residual tint. The silver halide system in glass lenses, by contrast, is regenerated with each cycle and is highly resistant to fatigue, one reason glass photochromic lenses were long valued despite being heavier than plastic ones.

Strength in Numbers

¹Most laboratory studies of bacteria have used cells swimming freely in liquid broth, a condition described as planktonic. In nature, however, many bacteria spend much of their existence attached to surfaces, living in dense communities called biofilms. A biofilm forms when free cells settle on a surface, multiply, and surround themselves with a slimy matrix of their own making, composed largely of polysaccharides, proteins, and DNA released from the cells. The film on unbrushed teeth, the slippery coating on river stones, and the deposits inside neglected drains are all biofilms.

²Biofilms are of more than academic interest because they readily form on medical devices. Urinary catheters, artificial heart valves, joint replacements, and contact lenses all provide surfaces on which bacteria can settle, and infections associated with these devices are notoriously difficult to cure. Bacteria living in a biofilm can survive antibiotic concentrations many times higher, in some cases a thousand times higher, than those that kill the same bacteria in planktonic form. Often the only reliable remedy is to remove the device entirely.

³Remarkably, this protection usually does not depend on genetic resistance. When bacteria are scraped from a biofilm and grown again as free cells, they are typically about as susceptible to antibiotics as the original population. Microbiologists therefore distinguish resistance, an inherited ability to grow in the presence of a drug, from tolerance, the ability to survive exposure to a drug for a time without being able to grow in it. Biofilm protection is largely a matter of tolerance, and several mechanisms contribute to it.

⁴The first is the matrix itself. For some antibiotics, especially those that carry an electric charge, the matrix acts as a barrier, binding or slowing the drug so that cells deep in the film are exposed to a lower concentration, or are exposed later, than cells at the surface. Yet this cannot be the whole story, since many antibiotics penetrate biofilms readily and still fail to kill the cells within.

⁵A second mechanism arises from the structure of the community. Cells near the surface of a biofilm have ample access to oxygen and nutrients, but those in the interior receive only what diffuses past their neighbors. As a result, much of a mature biofilm consists of cells growing slowly or not at all. Many antibiotics attack processes that are most active in growing cells, such as building the cell wall or making new proteins, so slow-growing cells are hard to kill.

⁶The third mechanism involves persisters, a small fraction of cells that enter a dormant state in which the targets of most drugs are largely inactive. Persisters occur in planktonic cultures too, but in a biofilm they are shielded by the matrix from the cells and antibodies of the immune system, which would otherwise clear them, so they can outlast a course of treatment. When the antibiotic is withdrawn, persisters can resume growing and rebuild the population, which helps explain why device infections often return after apparently successful therapy. Because persisters are not genetically different from their neighbors, their descendants are once again mostly susceptible to the drug.

⁷These findings have practical consequences. The standard laboratory tests that guide the choice of an antibiotic measure the lowest concentration of the drug that prevents the growth of planktonic cells. Such tests can suggest that an infection should respond to a drug that in fact fails against the biofilm in the patient's body. Researchers are therefore developing tests that grow bacteria as biofilms before exposing them to drugs, as well as treatments that break up the matrix, interfere with the chemical signals bacteria use to coordinate biofilm formation, or rouse persisters so that conventional antibiotics can act on them.

⁸For now, prevention remains the most effective strategy. In eye care, for example, contact lens storage cases are a common site of biofilm growth, and contaminated cases have been linked to serious corneal infections. Practitioners advise rubbing and rinsing lenses, emptying and air-drying cases between uses, and replacing cases regularly, measures intended to remove young biofilms before they can mature.

How the Eye Learns Its Length

¹Few organs change as much after birth as the eye. A newborn's eyeball is only about 70 percent of its adult length, and because it is so short, the image formed by its cornea and lens typically falls slightly behind the retina. Most infants are therefore mildly farsighted, or hyperopic. Over the first several years of life the eye lengthens, while the cornea and the crystalline lens become flatter and optically weaker. These changes are not independent of one another. Across a large group of children, refractive errors that are widely scattered at birth narrow toward a common value near zero, a state in which distant objects are focused sharply on the retina without effort. Vision scientists call this coordinated process emmetropization, and for much of the twentieth century it was regarded as a largely inherited program that simply unfolded on schedule.

²That view began to change when researchers noticed what happens to eyes that are denied a clear image early in life. Children born with a dense cataract, or with a drooping eyelid that covers the pupil, often develop eyes that grow too long, so that the image falls in front of the retina and distant objects appear blurred. This condition, nearsightedness or myopia, could also be produced deliberately in laboratory animals. When a translucent diffuser was placed over one eye of a young chick, that eye elongated rapidly and became measurably myopic within days, while the uncovered eye developed normally. The finding implied that eye growth was not merely programmed; it was steered by the quality of the visual image.

³Experiments with lenses showed how precise this steering could be. A negative, or diverging, lens moves the focused image backward, behind the retina. Chicks fitted with such lenses responded by accelerating the growth of the eye until the retina had moved back to meet the image. A positive, or converging, lens has the opposite effect, pulling the image forward, and chicks wearing positive lenses slowed their eye growth until the mismatch was again eliminated. Because the eye corrected errors of either sign, it could not simply be responding to the overall amount of blur, which is similar for images in front of and behind the retina. Somehow the developing eye detects the direction of defocus and adjusts its growth accordingly. Similar compensation has since been demonstrated in tree shrews, marmosets, and rhesus monkeys, although mammals respond more slowly than birds.

⁴Where in the visual system is this decision made? The obvious candidate was the brain, which interprets blur in conscious vision and controls the focusing muscles of the eye. Two lines of evidence pointed elsewhere. First, when a diffuser covered only the upper or lower half of the visual field, only the corresponding half of the eye grew abnormally, producing a lopsided globe. A signal routed through the brain would be expected to affect the whole eye. Second, chicks whose optic nerves had been cut, severing communication between eye and brain, still compensated for lenses in roughly the right direction. The analysis of the image, and the command to grow, appear to originate largely within the retina itself.

⁵The retina, however, does not lengthen the eye directly. It sends chemical signals outward through the layers behind it. The first tissue to respond is the choroid, a spongy, blood-rich layer between the retina and the tough outer coat of the eye, the sclera. In chicks wearing positive lenses, the choroid can thicken substantially within hours, physically pushing the retina forward toward the image; with negative lenses it thins. These choroidal shifts are a quick but partial correction. The lasting change comes more slowly, as cells in the sclera alter the production and breakdown of the structural molecules that make up the wall of the eye, allowing the wall to expand or restraining its growth over a period of weeks.

⁶Among the candidate messengers, dopamine has received particular attention. Certain retinal neurons release dopamine in response to light, and its release follows a daily rhythm, rising during the day. In chicks, the dopamine content of the retina falls in eyes that are deprived of clear vision, and drugs that activate dopamine receptors can suppress the excessive growth that a diffuser would otherwise cause. These observations do not prove that dopamine is the only stop signal for eye growth, but they made it a natural link between the visual environment and the machinery of growth.

⁷The question has become pressing because myopia is no longer uncommon. In parts of East Asia, surveys of young adults finishing secondary school have found that as many as eight or nine in ten are myopic, compared with a small minority a few generations earlier. Prevalence has also climbed in Europe and North America, though less steeply. Change this rapid, occurring within the span of a few generations, cannot be explained by shifts in the gene pool. Genes clearly influence individual susceptibility, since children of myopic parents are at higher risk, but the speed of the increase points to changes in how children live. Two environmental factors stand out in study after study: the intensity of education, with its long hours of reading and close work, and the amount of time children spend outdoors.

⁸Of the two, time outdoors has proved the more surprising. Children who spend more time outside are less likely to become myopic, and the association remains even when researchers account for how much reading and close work the children do. Nor is it explained by exercise, since time spent playing sports indoors shows little protective effect. School-based trials, in which classes were given extra time outdoors each day, found fewer new cases of myopia among the participating children than among children in comparison schools. The leading explanation is light intensity. Even in shade, daylight commonly provides ten or more times the illumination found in a typical classroom, and in direct sun the difference can exceed a hundredfold. In animal studies, chicks reared under very bright light develop less deprivation myopia than chicks kept under ordinary laboratory lighting, and this protection is weakened when a drug that blocks dopamine receptors is given, consistent with the idea that bright light acts through retinal dopamine.

⁹Light is not the only candidate. Outdoors, most objects are far away, so images across the whole visual field lie near a common focus. Indoors, nearby walls, furniture, and books place parts of the scene at very different distances, and some researchers argue that the resulting pattern of defocus across the retina, especially in its periphery, encourages the eye to grow. The two explanations are not mutually exclusive, and both may contribute. What does seem clear is that outdoor time works best before myopia begins: it reduces the number of children who become myopic but appears to do comparatively little to slow the condition once it is established.

¹⁰For children who are already myopic, other approaches aim to slow progression. Eye drops containing a low concentration of atropine have reduced the rate of progression in clinical trials. Atropine blocks muscarinic acetylcholine receptors and, at high doses, paralyzes the muscle that focuses the eye for near vision, so it was once assumed to work by relieving the strain of close work. That explanation has lost favor, partly because atropine also inhibits experimental myopia in chicks, whose focusing muscle is controlled through a different type of acetylcholine receptor that atropine does not block. Its site of action is now thought to lie in the retina or in the tissues behind it. Specially designed spectacle lenses and contact lenses take yet another approach, keeping central vision sharp while deliberately placing images in front of the peripheral retina, in effect delivering the same "slow down" signal that a positive lens gave the chicks.

¹¹These efforts matter because myopia is more than an inconvenience solved by glasses. Glasses and contact lenses correct the optical error but do not change the shape of the eye, and an eye that has grown excessively long has a stretched, thinned retina and sclera. Highly myopic adults face substantially higher lifetime risks of retinal detachment, glaucoma, and degeneration of the central retina, conditions that can permanently impair sight. Viewed in this light, myopia is less a simple refractive error than a disorder of growth, one whose course is shaped in childhood by how much light, and what kind of visual world, the developing eye receives.