IELTS Academic Reading Practice Test 91

The mirror is now such an ordinary object that we rarely consider how it was produced or what it does. Yet for most of human history, obtaining a clear reflection was far from easy. The earliest known manufactured mirrors were small pieces of polished obsidian, a dark volcanic glass, found at sites in Anatolia and dated to about 6000 BCE. These objects were too small and dark to be used for everyday grooming, and archaeologists believe they probably served as personal adornments or ceremonial items rather than as practical aids to appearance.

By the third millennium BCE, Egyptian and Mesopotamian craftsmen were making mirrors from polished metal, especially copper and later bronze. These early mirrors were typically round, with a polished front surface and a decorated back or handle, and they had to be polished frequently because the metal tarnished quickly. Examples have been excavated from tombs and temples, suggesting that mirrors were valued possessions. The Romans later produced larger mirrors of polished silver, and during the first century CE glass mirrors with a lead backing also appeared, though they remained luxury items and were not widely available.

After the collapse of the Roman Empire, metal mirrors remained the standard across Europe for many centuries. A decisive step was taken by Venetian glassmakers, who were concentrated on the island of Murano and had learned to produce unusually clear, flat glass. They coated this glass with an amalgam of tin and mercury, creating mirrors of a brilliance that had never been seen before. By the sixteenth century, Murano mirrors were prized throughout Europe, and the demand for them was so great that rival governments tried to persuade Murano craftsmen to emigrate. The Venetian authorities guarded the technique so closely that craftsmen who tried to leave the city faced severe penalties, but the secrets nevertheless spread gradually, and production began in France and England.

A further transformation came in 1835, when the German chemist Justus von Liebig found a way to deposit a thin layer of metallic silver onto glass. He reduced an ammoniacal solution of silver nitrate with a sugar, producing a bright, even coating. This silvering process made mirrors far cheaper and more durable than earlier types, and it quickly replaced the mercury method. Silver, however, tarnishes over time, so many modern mirrors use aluminium, which is more resistant to corrosion.

Today many mirrors are manufactured by taking a sheet of high-quality float glass and placing it in a vacuum chamber. Inside this chamber, a thin film of aluminium is evaporated onto the back surface of the glass, after which the metal layer is protected with several coats of paint. The process is fast, inexpensive and capable of producing mirrors with very high reflectance. Although aluminium reflects slightly less than silver, its resistance to tarnishing makes it preferable for most purposes. Aluminium mirrors of this kind are used in bathrooms, vehicles and optical instruments alike.

A mirror reflects light because the metal coating conducts electricity and interacts with the incoming light waves, absorbing their energy and re-emitting it as a reflected wave. In a standard household mirror, the reflective metal is on the back of the glass. Light therefore passes through the glass, reflects from the metal, and travels back through the glass to the viewer. The glass protects the delicate metal coating, but it also creates a faint secondary reflection. Precision mirrors, such as those used in lasers, place the reflective coating on the front surface to eliminate this secondary image. The wave theory of light explains this behaviour, but the practical result is simple: a smooth metal surface returns a clear image.

Not all mirrors are flat. Concave mirrors curve inward and can focus light or produce magnified images, which makes them useful in shaving mirrors and reflecting telescopes. Convex mirrors, by contrast, curve outward and give a wider field of view, so they are often installed as safety mirrors at road junctions and in shops. A one-way mirror, sometimes called a half-silvered mirror, has a metal coating that is extremely thin: it reflects most of the light that strikes it but allows some to pass through. This effect works only when one side of the glass is brightly lit and the other is dim, which is why such mirrors are used in observation rooms. Different curvatures therefore produce different image sizes and fields of view, and a single building may contain several types.

Mirrors have also played a significant role in psychology and culture. In the 1970s, the psychologist Gordon Gallup devised his famous mirror test, in which a mark is placed on an animal's body and the animal is given access to a mirror. If the animal touches the mark on its own body rather than on the image, it is said to show self-recognition; children usually pass the test at around eighteen to twenty-four months.

The test has been used with animals such as chimpanzees, elephants and dolphins, although its interpretation remains debated. More broadly, mirrors appear in myth and literature, from Narcissus to Snow White, as symbols of vanity, truth or dangerous self-absorption. The ordinary act of checking one's reflection thus depends on a long history of technical skill and rich symbolic associations.

In 1989, the British epidemiologist David Strachan published a paper that would shape decades of research into allergies. He observed that children with older siblings were less likely to suffer from hay fever, and he suggested that infections acquired in early childhood, particularly those passed on by older brothers and sisters, might somehow protect against allergic diseases. This idea became known as the hygiene hypothesis. At its simplest, it proposed that the modern reduction in childhood infections was responsible for the rise in allergies such as asthma and eczema.

Strachan's original emphasis on infections proved too narrow. Later researchers argued that the critical factor was not disease-causing infections themselves, but the ordinary, harmless microorganisms that humans encountered throughout most of their history. The immune system, according to this revised view, evolved in the presence of a rich array of bacteria, viruses and parasites, and it depends on such encounters to learn how to regulate its responses. Without them, the immune system may overreact to otherwise harmless substances, producing the inflammation seen in allergic conditions. In other words, the immune system's regulatory networks are shaped by these early encounters, and a lack of microbial stimulation can leave them prone to exaggerated responses.

Epidemiological evidence has provided support for this broader version. Children who grow up on farms, particularly those who have regular contact with livestock and who drink unpasteurised milk, tend to have markedly lower rates of asthma and allergies than children in non-farming rural areas or cities. Studies from Alpine regions, where traditional farming is still common, have repeatedly reported this protective effect. Similar patterns have been seen among children who attend day care from an early age and among those with several older siblings, both of which increase exposure to common microbes.

Much of this research has shifted attention to the gut, where trillions of bacteria live. These intestinal microbes are not passive passengers; they help digest food, produce vitamins and, importantly, train the immune system to distinguish between threats and harmless substances. Graham Rook, a British immunologist, has proposed that humans co-evolved with a range of organisms he calls 'old friends', including certain gut bacteria and parasitic worms. According to Rook, modern sanitation and medical treatments have reduced our exposure to these organisms, leaving the immune system poorly calibrated. His 'old friends' framework emphasises that some organisms that were once common in the human digestive tract are now absent in many urban populations.

The modern environment differs from that of our ancestors in several ways. Clean drinking water, improved sanitation and food safety have greatly reduced the burden of infectious disease, which is an enormous benefit. At the same time, children spend more time indoors, antibiotics are used more frequently, and births by caesarean section are more common. Each of these changes can alter the range of microbes a child encounters in early life. Some researchers argue that this reduced microbial diversity, rather than cleanliness as such, is the key problem.

The hypothesis has also been used to explain the rise of autoimmune diseases, in which the immune system attacks the body's own tissues. Conditions such as type 1 diabetes, multiple sclerosis and inflammatory bowel disease have become more common in wealthy countries over recent decades. Some scientists suggest that the same lack of early microbial exposure may be involved, because these diseases, like allergies, involve abnormal immune regulation. However, the evidence here is largely indirect, and proving a causal link has been difficult. Although this idea is attractive, the temporal trends do not always match the predictions, and other environmental changes may also be responsible.

Critics point out that the hygiene hypothesis is often misunderstood. It does not claim that children should be exposed to dangerous infections or that handwashing is harmful. The word 'hygiene' is misleading, since the relevant idea concerns microbial exposure, not personal cleanliness. Furthermore, not all studies have confirmed the predicted patterns, and the rapid rise in allergies cannot be fully explained by any single factor. The hypothesis is best seen as one part of a complex picture that includes genetics, diet and air pollution. For example, increased time spent indoors reduces not only exposure to soil and animals but also physical activity and sunlight, both of which are known to influence immune function.

The implications of the hypothesis are still debated. Public health authorities continue to stress the importance of vaccination, handwashing and food safety. At the same time, researchers are investigating whether interventions such as probiotic supplements or exposure to farm dust might help prevent allergies without increasing the risk of infection. For parents, the practical message is to avoid unnecessary antibiotics and allow children to play outdoors, while maintaining normal hygiene. The scientific goal is to distinguish harmful microbes from those that may actually be beneficial. Researchers are still working to identify which specific organisms confer protection and how they do so.

Every school claims to value good teaching, yet there is surprisingly little agreement about what makes a teacher expert. Some people believe that expertise comes naturally with years in the classroom; others point to academic qualifications, while school inspectors often focus on measurable outcomes such as test scores. In my view, although experience is necessary, it is not sufficient. The research suggests that expert teaching depends on a particular combination of knowledge, deliberate practice and the ability to adapt to learners. This is why attempts to define expertise have proven so difficult: teaching is a complex, context-dependent activity that cannot be reduced to a single quality.

The commonsense assumption that longer experience automatically produces better teaching is not supported by evidence. Several studies have found that the performance of teachers often improves during the first few years and then levels off, with little further automatic gain. The cognitive psychologist K. Anders Ericsson, whose work on expertise is widely cited, argued that improvement comes not from repetition but from what he called deliberate practice: a focused effort on specific weaknesses with immediate feedback. Teachers who simply repeat the same lessons year after year may become fluent but not necessarily more effective. This is not to deny that some experienced teachers are excellent, but their excellence is the result of something more than time. In short, years in the classroom alone are not a reliable guide to teaching quality.

One important ingredient is what the American educational researcher Lee Shulman called pedagogical content knowledge. This is not merely knowing a subject, nor merely knowing general teaching techniques, but understanding how to present particular ideas in ways that learners can grasp. A chemistry teacher needs to know not only chemistry but also the misconceptions students bring to the topic and the analogies, demonstrations and questions that can correct them.

Shulman's work, which began in the 1980s, shifted attention from generic classroom skills to the intellectual demands of teaching specific subject matter, and it remains influential in teacher education today. For example, research on physics teaching has shown that even undergraduate students often hold deep-seated misconceptions about force and motion. Expert teachers develop this kind of knowledge by reflecting on repeated teaching of the same topics, and they can draw on a store of examples and questions.

Expert teachers also appear to see classrooms differently. Research using eye-tracking and video analysis has shown that experienced teachers notice events that novices miss, particularly signs of confusion, disengagement or low-level disruption. They do not necessarily have faster reactions; instead, they have built mental routines that allow them to interpret classroom cues quickly and decide which ones require action. This kind of situational awareness develops over time, but only when teachers actively reflect on their practice rather than merely accumulating hours. Novices can be trained to improve their noticing through structured video review, which suggests that this skill is not entirely automatic.

Another characteristic is adaptive expertise. In medicine and engineering, researchers distinguish between routine experts, who solve familiar problems efficiently, and adaptive experts, who can handle novel situations and learn from them. The same distinction applies to classrooms. A teacher who can keep order with a well-behaved class may struggle when faced with unexpected difficulties. Adaptive experts not only use proven strategies but also question them, adjust their plans in response to feedback from students, and seek out new approaches when old ones fail. This flexibility is especially important in schools with highly diverse student populations. Adaptive expertise is therefore a key feature of expert teaching.

The policy implications are significant. Systems that judge teachers solely by their students' test results, or that reward years of service above all else, may fail to develop genuine expertise. By contrast, structured professional development that includes observation, feedback and coaching can help teachers improve at any stage of their careers. Such programmes are expensive, but the evidence suggests that well-designed mentoring produces larger gains than many broader reforms. However, I would caution against treating expertise as a checklist: a teacher can demonstrate all the right behaviours and still lack the sensitivity to respond to a particular child. In countries such as Singapore and Japan, lesson study groups provide regular opportunities for collaborative planning and peer observation, and these have been linked to sustained teacher improvement.

Some researchers have tried to measure teacher expertise through value-added scores, which estimate how much a teacher contributes to students' test gains. While these scores can identify some consistently high-performing teachers, they are statistically noisy and can change from year to year. More fundamentally, they capture only one dimension of teaching. An expert teacher may inspire a love of reading, support a struggling student emotionally, or design a project that develops critical thinking, none of which may appear in test results. The use of value-added measures for high-stakes decisions is therefore controversial.

What, then, should we look for? In my judgement, an expert teacher is someone who combines deep subject knowledge with the ability to make that knowledge accessible, who reflects systematically on their own teaching, and who adapts flexibly to the needs of different learners. Such teachers are not produced automatically by time, nor can they be manufactured by a set of generic competences. They are developed through a career-long process of deliberate practice, feedback and thoughtful engagement with students. Ultimately, expertise in teaching is less a fixed state than a continuous pursuit of improvement. It is therefore more useful to focus on the processes that develop teachers than on static lists of traits.