IELTS Academic Reading Practice Test 450

Antarctica in winter is the least hospitable environment on Earth. For months the sun never rises above the horizon, air temperatures drop below minus fifty degrees Celsius, and gales sweep across the sea ice, so that almost every animal that lives on the continent, from seals to seabirds, migrates northwards long before the darkness sets in. The emperor penguin does not. It remains on the ice all winter and is the only bird in the world that breeds in these conditions. Even when blizzards bury the colony, it incubates a single egg while the nearest open water lies many kilometres away.

The emperor is the largest of the penguins, standing about 1.1 metres tall and weighing between 22 and 45 kilograms. Its black back and white front are familiar, but far less obvious are the patches of yellow-orange plumage behind the eye and on the upper breast. The species is distributed all around the Antarctic coastline, where several dozen colonies have been recorded, most of them on fast ice, sea ice frozen firmly to the coast, rather than on ice that floats freely, breaks up in storms and can be carried hundreds of kilometres northwards by currents.

The breeding cycle is tightly organised around the seasons. Adults arrive in March and April, walking or sliding over the frozen surface, sometimes for more than a hundred kilometres from the edge of open water. They gather at traditional sites, some of which have been used for centuries, and after an elaborate period of courtship the pairs form. In May or June the female then lays a single egg weighing around 450 grams, passes it carefully to her partner and leaves at once for the ocean, where she will feed on fish and squid for roughly two months before returning to the colony.

The transfer of the egg is delicate. It must not touch the ice for more than a moment, since the surrounding air is far below freezing and the embryo would quickly perish. The male balances the egg on his feet and covers it with a fold of warm skin, the brood pouch, which lies just above them. Once the egg is secure, the female waddles to the perimeter of the colony and begins the long journey to open water across ice that may stretch for many kilometres.

For the male, the weeks that follow are a feat of endurance. He cannot leave the egg to feed, because it would freeze within minutes, and he has no food on the ice. Investigations at colonies have established that he may go without eating for more than a hundred days, during which his body slowly consumes the fat laid down during the summer. By the time the chick emerges he may have lost close to half his starting weight, while the females are out at sea, rebuilding their own reserves.

The males survive the cold partly by cooperating. They gather in dense huddles, in which each bird presses against its neighbours and bends its head down, so that only a rounded back is exposed to the wind. Measurements taken inside such groups have recorded temperatures far higher than those outside, yet a huddle is not a static mass. Birds on the windward edge, which suffer most from the wind, shuffle gradually towards the centre, while those in the middle are pushed outwards, so that over several hours each bird drifts through the group and no individual remains on the exposed edge for long.

When the egg hatches, the female usually returns within a few days, her belly full of fish and squid, which she brings up to feed the chick. The male then hands over the young bird and heads for the sea himself, often having lost so much weight that he can barely walk. From this point the parents take turns, one guarding the chick while the other forages, and because emperors build no nests the chick stands on its parent's feet.

By August and September the chicks demand more food than one parent can bring, so both adults now go fishing. The young gather in crèches, groups that provide warmth and protection while their parents are away. Towards the end of the year the downy chicks begin to lose their grey coat and grow the waterproof feathers they will need at sea.

Fledging takes place in December or January, when the sea ice begins to break up. The young birds, now almost as large as their parents, enter the water for the first time and disperse. Little is known about where they go after fledging, though some young birds have been tracked travelling thousands of kilometres across open ocean, and those that survive are thought to return to the colony where they hatched at about four or five years of age.

Emperors are formidable swimmers. They dive to depths of more than 500 metres in pursuit of fish, squid and krill, and can remain submerged for over twenty minutes, slowing their heart rate as they descend. Their main enemies at sea are leopard seals and killer whales. On the ice, skuas and giant petrels take eggs left unattended and small chicks that stray from their parents during a blizzard or a change of shift.

Counting emperor penguins is difficult, since colonies are remote and often in darkness. Satellite images that detect the reddish-brown stain of guano on the ice have revealed colonies that were previously unknown to science. Estimates of the total population, which may number several hundred thousand birds, remain approximate because so many colonies are rarely visited.

A rainbow looks like an object in the sky, but it is nothing of the kind. It is an optical effect that appears when sunlight enters countless water droplets suspended in the air, and it exists only for an observer in the right place. Move a few metres and the bow moves with you; nobody can ever reach its end. The same geometry explains the arcs seen in spray from a garden hose, in the mist above a waterfall and, on rare occasions, in moonlight.

The path of a light ray through a raindrop was worked out in the seventeenth century. René Descartes, better known for his philosophy, calculated the route in an appendix to his Discourse on Method in 1637, treating the drop as a sphere and ignoring the way real light behaves as a wave. A ray of sunlight strikes the curved surface and is bent, or refracted, as it passes from air into water. It then travels to the far side of the drop, where part of it is reflected back towards the front. As it leaves, it is refracted a second time.

Two effects combine to produce the colours. Water bends different wavelengths through slightly different angles, so white sunlight is fanned out into a spectrum, with violet bent most and red least. In addition, because the drop is a sphere, rays entering at different points are turned through different angles. Most of these rays spread out and never reach the eye. But rays entering close to a particular angle emerge crowded together, producing a bright band, and it is this concentration of light that we see as the bow.

For the primary rainbow, the brightest, the favoured angle is about 42 degrees from the direction opposite the sun. That is why a bow always appears with the sun behind the observer, and why it is never seen when the sun is high in a clear summer sky: if the sun stands more than about 42 degrees above the horizon, the bow falls below the horizon. From an aircraft, or from a hill above mist, a complete circle is sometimes visible. On the ground we see only the upper part of that circle, cut off by the horizon.

The colours of the primary bow run from red on the outside to violet on the inside. Their order is not arbitrary but follows from the way water bends light: red emerges at a slightly larger angle than violet, so red appears on the outer edge. The band is not sharp, and between the colours there are gradual transitions. Beyond the red edge the bow ends abruptly, since no light reflected once inside a drop can emerge at a larger angle. Inside the violet edge, by contrast, the light is spread over a wide range of angles and is correspondingly weak.

A second, fainter bow often appears outside the first, with its colours reversed. It is produced by rays that undergo two internal reflections before leaving the drop, which turn them through a larger angle, around 51 degrees. Each extra reflection loses light, so the secondary bow is noticeably dimmer. The region between the two bows is darker than the rest of the sky, a phenomenon named Alexander's band after the ancient Greek writer Alexander of Aphrodisias, who described it.

The simple ray model leaves some features unexplained. On the inner edge of a bright primary bow, observers with good eyesight sometimes see a series of faint pastel arcs, known as supernumerary bows. These arcs are an interference effect: light that has taken slightly different paths through the drop meets again and, depending on the difference in distance, reinforces or cancels particular colours. Their spacing varies with the size of the raindrops. Thomas Young had attributed them to interference in the early 1800s, and the English astronomer George Biddell Airy developed a fuller mathematical treatment of the problem in 1838.

The size of the droplets matters. Large drops, around a millimetre or two across, give sharp, vivid bows. Smaller drops produce paler and broader ones, and when the droplets are extremely small, as in fog, no colours appear at all: the result is a white arc, sometimes called a fogbow. This is one reason why rainbows are brightest when rain is heavy and the drops are large, often towards the end of a shower as the sun comes out behind it. Seen from a distance, a bow in fine drizzle can look almost colourless.

Rainbow light is also strongly polarised, a fact established by physicists in the early nineteenth century. Light reflected inside the drop emerges vibrating mostly in one plane, so a bow largely disappears when viewed through polarising sunglasses held at the right angle. Photographers sometimes use a polarising filter to make a bow stand out against the clouds, or to remove it altogether by rotating the filter.

A complete account of what happens when light meets a water drop requires a theory that treats light as a wave and allows for the drop's true shape and size; the German physicist Gustav Mie published such a solution in 1908. Even so, the simple geometry that Descartes set out nearly three centuries earlier remains the best way to understand why a rainbow appears where it does. The same geometry explains why the sky inside the primary bow looks brighter than the sky outside it.

Few decisions shape a child's experience of school more than the way classes are organised. In some countries, pupils of a similar age are taught together regardless of attainment; in others, they are divided into sets or streams, often after tests taken at eleven or twelve. The practice is known in different places as streaming, tracking or setting, and it remains one of the most contested questions in education. Supporters speak of pace and challenge, critics of labels and lost opportunities. The argument has continued for the better part of a century, and it shows little sign of being resolved.

The arguments for separating pupils by ability are, on the face of it, straightforward. A teacher facing a class whose attainment spans five or six years must choose between moving too slowly for the quickest and too fast for the slowest. Grouping pupils of similar attainment allows a teacher to pitch a lesson at one level, cover material faster with the ablest and give weaker pupils more time and support. In England, setting remains common in secondary mathematics and science, and many teachers defend it on exactly these grounds.

The evidence is less clear-cut than such arguments suggest. Reviews of the research by the American educationalist Robert Slavin, covering many studies from several countries, found that grouping pupils by ability produced, on average, almost no difference in achievement. What mattered was how the groups were taught. Where teachers adjusted their methods to the class in front of them, overall results were much the same; where they did not, the gap between groups tended to grow. The finding has been repeated in several later reviews.

A second line of evidence concerns what happens inside the groups. Studies in several countries have found that pupils placed in lower sets are more likely to be taught by less experienced teachers, to experience a slower pace, to be given repetitive tasks and to have fewer opportunities for discussion and problem-solving. Being placed in a low group at an early age also affects how pupils see themselves, and once a judgement has been made it tends to stick: movement between sets is limited, and pupils rarely move up.

International comparisons point in a similar direction. Analyses of the OECD's PISA survey have repeatedly found that education systems which separate pupils by ability at an early age show a wider gap between their highest and lowest achievers than systems that delay selection. Nor do they show any clear advantage in average performance. The pattern has appeared in successive rounds of the survey. Such correlations cannot prove that selection causes inequality, since countries differ in many other ways, but they are hard to reconcile with the claim that early setting raises standards overall.

Defenders of setting reply that average figures conceal individual cases. A gifted mathematician in a mixed class may spend much of the year waiting while others catch up, and the boredom that follows is a real cost. It is also argued that mixed-ability teaching places an unreasonable burden on teachers, who must prepare several versions of every task and monitor pupils working at different levels simultaneously. In crowded classrooms with limited resources, such demands may simply be unrealistic.

The way a school organises its classes is only one factor among many. Class size, the quality of teaching, the curriculum and the level of support for pupils with difficulties are all likely to matter more, and variations between schools within the same national system are often greater than variations between systems. The evidence suggests that a badly implemented version of either approach will fail, while a well-implemented version can succeed.

It is also worth distinguishing between kinds of grouping. Teaching a whole class together and expecting every pupil to do identical work is not the same as teaching a mixed class with planned variation. In the latter, pupils might work in small, flexible groups that change from week to week, with the teacher providing support where it is needed and extension tasks for those who are ready. This approach, sometimes called within-class grouping, is more demanding of teachers but has better support in the research than permanent sets.

My own view is that mixed-ability classes should be the default, particularly for younger pupils. The reasons are partly about fairness: separating children at eleven on the strength of a test measures their background and prior schooling as much as their potential, and the labels attached are difficult to shake off. The arguments are also practical. A school that places pupils in fixed sets early gives itself little room for a pupil who develops late, and it concentrates disadvantaged pupils in the groups that receive the least experienced teachers.

None of this means that mixed-ability teaching can be done cheaply. It requires well-trained teachers, sensible class sizes and a willingness to group pupils differently for different purposes, sometimes within a single lesson. Where those conditions are absent, setting may be the least bad option, and it seems reasonable to allow it in the final years of school for subjects in which the range of attainment becomes very wide. The question, then, is not mixed ability or setting, but what a school is able and willing to do to make either work.