Answers to Last Issue's Do You Know? 1. According to my science textbook, sunlight is white. Then why does it appear yellow to the eye? Answer: Let us first start with a fact: white light is a combination of all colours produced equally by a glowing object. If that glowing object produces more blue light than it produces red, orange, yellow or green light, then it appears blue. Why does it produce blue or orange light at all? That depends on its temperature. Now we can proceed to answer the question. The two reasons why the sun appears yellow are: (a) The surface temperature of the sun is 5,500 degrees C. This produces a range of visible light (red to blue) in which yellow is the largest, but not much more than the other colours it produces. If the sun were cooler, say 2,500 degrees C, it would look red, like the stars Antares and Betelgeuse. Or if the sun were hotter, say 15,000 degrees C, it would look blue, like the star Rigel. (b) The Earth's atmosphere acts as a kind of light filter, and some colours are filtered more than others. The sun is a yellow star, but if you were to observe it from space where it would appear more white than yellow. It is the Earth's atmosphere makes the sun look more yellow. Actually you do not need to go into space to see the difference. If you are in the Himalayas at 4000 metres elevation, the sun would look less yellow and more white than at sea level. This is because there are fewer air molecules at this elevation to filter the sun's other colours. Imagine how the sun would look from an airplane at 10,000 metres altitude: fully white. There is another aspect to consider. For us it is easier to look at the sun in the morning or the afternoon, even if only for a few seconds, than at noon. The sun appears more yellow at those times than at noon when it looks at its brightest and whitest. The reason is that because of the sun's high position at noon, the sunlight has less air to travel through. Less air means less filtering of other colours. (Please do NOT look at the sun directly, it is dangerous.) Remember: light appears white because all colours are equally reaching your eyes. So, at noon the sun appears to be more white, less yellow: closer to the way it really is! It is also relevant to recall why the sky appears blue: the short wavelengths (blue) of light from the sun are scattered by the atmosphere. ------------ 2. How is a hissing sound produced when water falls on a hot surface? Answer Ask yourself first how you hear any sound at all. Sound propagates by a pressure wave: this happens by the to and fro motion of particles of the medium. When an object vibrates, the particles around the medium vibrate. The particle in contact with the vibrating object is first displaced from its static position. Each particle disturbs the other particle in contact. This sets up the pressure wave. When water (or in general, when a liquid heats up) its vapour pressure increases. It is the pressure exerted by the particles in a liquid by their kinetic energy (got from heating) in opposition to atmospheric pressure. When the vapour pressure increases to a value greater than the atmospheric pressure, boiling occurs. At what is called one atmosphere of pressure, this occurs at 100 degrees centigrade. When water is poured on a surface that is sufficiently hot to heat water to its boiling temperature very quickly, it appears instantaneous to us. Then we can imagine that there would be a very hot local environment around the surface. The temperature then falls off so fast as you move away from the surface, that you can even say that the temperature and pressure are not affected by the surface at all. But at the same time the heat is enough to further heat, and increase the pressure of the water vapour boiling off of the surface. This rapid increase in pressure relative to the atmospheric pressure creates a pressure gradient which propagates as a sound wave, which is why you hear sound from the boiling water. You do not hear the same sound from a pot of boiling water because we do not have the same rapid heating of water vapour creating the large pressure difference between vapour and the atmosphere. ------------ 3. Many websites have a "Captcha" to check that a human being is using it. How does this work? Answer This is indeed becoming more and more important every day. The Internet has made a big difference in modern life. Especially during the times of national lockdowns to stop virus spreading, online transactions have become important. Students have online classes. Many of these services intended for human beings need a way to ensure that only human beings are using them. That is where these "Captcha" tests come in. A natural question to ask: if it is not human beings using the services, who can it be? How can something inhuman use a "service"? The answer is simple: other computer programs. To understand this, we need to understand that anything that we use on a computer (or application on a mobile phone) is actually a program. It takes "input" from the user and responds. As far as that program X is concerned, any source of input is the user. That input could come from another program Y, and the program X cannot tell. This may seem stupid but this is what makes computers extremely flexible, connect to anywhere, disconnect at will. By programs connecting to other programs, we have built the Internet. That a program does not distinguish a device, a human being and another program is actually a great source of strength, an important principle in computer science. Unfortunately this comes at a cost: some services are meant only for human beings, where we wish to disallow devices and programs. To cheat these services, some programmers have created "bots" that pretend to be human users and access secrets, commit frauds. The most common way this differentiation of bots and humans is done today, is the CAPTCHA or the single click on a box. How can this help? Before answering this, we should also ask what harm can bots do. They can be trained to do many bad things. Bots can create multiple accounts on social networking platforms and email providers (like Gmail), thus inflating the number of users and creating havoc elsewhere on the internet with these email accounts. They can fill in forms with unwanted content and spread nonsense. They can post nasty comments on websites. Worse, they can collect people's passwords. Hence the need to distinguish humans and bots is serious. As it happens, this was a question asked long before actual computers were built, by someone who started the study of computers mathematically. Alan Turing, the British mathematician on whom the Hollywood film "The Imitation Game" was based, asked this question in 1950. He suggested that an intelligent program is one that passes the imitation test: a human observer, who submits questions should not be able to tell whether the answer is coming from another human being or from the program. This initiated the field of Artificial Intelligence and many researchers have been studying this problem. Among these studies one led to CAPTCHA, short for “Completely Automated Public Turing test to tell Computers and Humans Apart”, developed by American scientists in 2000. It was a way to filter out unwanted bots from websites by using distorted images, puzzles, audio transcription, etc. This method has been used to monitor credit card fraud by the program PayPal. The idea is that programs find it hard to decipher distorted visuals, whereas humans can easily decode them. At one point in time, this CAPTCHA method was being used by 200 million users every day, which amounts to spending approximately 500,000 hours transcribing scrambled text! In fact, some scientists decided to turn all this effort into something useful and used this method of bot detection to digitize classic books. This new method was called the ‘reCAPTCHA’ and it used scanned books and other materials as distorted tests to make users transcribe them, which solved two problems at the same time, eliminating bots and digitizing classic books. Google bought this technology in 2009 and started developing it. In 2014, Google released a scientific paper stating that it had developed image recognition systems that are capable of solving the hardest CAPTCHAs with 99.8% accuracy, which made the current system unreliable. So things have changed now. In December 2014, Google announced that it had developed a new version of reCAPTCHA: the one that is common today, the "I am not a robot" click box. This version does not make the user transcribe the distorted text, but instead figures out with just one click if you are a human or bot. Th method analyzes what the user was doing before, during and after clicking the box, including how you move the cursor. The CAPTCHA has not completely been replaced. Instead there are images of, say a cat, or traffic lights, or cars, that the user must identify among other options. This method is also a boon for people with visual impairments, as it reduces the time it takes to transcribe and replaces it with just a clicks. It is safe to say that this is an ongoing battle, between bot makers and bot breakers. At the moment the re-CAPTCHAs and click boxes are doing better, but before long we can expect superior bots that can get around them, leading to another escalation. ------------ 4. In many competitions, we often "root for the underdog", that is, support someone who would ordinarily be expected to lose. Why do we do this? Answer One simple reason for this is that it makes the competition more exciting. There is clearly no fun in any one-sided, predictable game. We want evenly matched players, and if one of them is clearly expected to lose, we would rather have that person be resilient and do better, just so that we have a matched game. There is another reason why we support underdogs: in many ways, we feel that we are very much like them. Everyone likes to win in life, confront all our challenges, and emerge victorious. But we are not always that fortunate, and understand that one has to struggle. Deep inside us, we know how it feels to have everything going against us, while others may seem to be getting things easily. Moreover, there is always a part of us that believes that we are more passionate and resilient than those who have an inherent advantage. So we can relate to the underdog, identify with their struggle, their passion and their resolve to push forward, despite being in a disadvantaged position. Another reason: watching the underdog persist through difficult times and succeed gives us the hope that we can perhaps succeed too. There is a deeper reason that social psychologists talk of: we all have a deep concern for justice and fairness within us. When we see the history of two teams, we might notice that the "top dog" has had far more victories than the underdog. Therefore, without even realising it, we introduce a parameter of fairness into our judgment, and conclude that things must have been unfair for the underdog. This is especially true if we see the underdog putting in very strong effort. Finally there is also expectation. We do not have high expectations from the underdog, so we would not be too disheartened if they lose. However, if they win, the pleasant surprise would make us extremely happy. Thus, we cannot lose much by being on the underdog's side. On the other hand, we have more to lose by supporting the top dog. When a top dog wins, it does not thrill us nearly as much; it was expected of them anyway. However, if they happen to lose, their unexpected loss could leave us shattered. All these are plausible explanations, in the sense they can all be right, in different contexts, for different persons. Why X supports the underdog on a specific day in a specific event is not some thing we can answer in general. ------------ 5. I always see wind coming from somewhere. Where does wind come from? Answer As I write this in Chennai on a June afternoon, I have strong breeze blowing -- so much so that I have to shut the windows for fear of papers flying. The stiff breeze is invigorating, but we rarely to pause to wonder where it is coming from, and why. Wind is a localized phenomenon, but in reality, it is a byproduct from huge global cycles of pressure, temperature and moisture. At the most basic level, wind is born because of differences in Earth's temperature, which are caused by the uneven heating of our planet by the sun. Another crucial factor behind wind is the air itself, the atmosphere around us, which is composed of nitrogen, oxygen and water vapour. Wind is caused by the motion of air from areas of high pressure to areas of low pressure. Air pressure is defined as the amount of force that the air exerts on a specific area. When a large mass of air is warmed by the heat of the sun, it begins to expand, become lighter, rises and disperses, thus exerting less pressure on the area. If the air that rises happens to be full of water vapour, then its movement can also create clouds, storms and adverse weather conditions. Conversely, there are also high-pressure areas surrounding these warmer bubbles; when these low-pressure conditions are created and the air rises, the high-pressure air (cooler and drier) will rush in to fill the empty space above the ground. This movement of air is the atmosphere's attempt to balance itself and find stability, but in the process, wind is created. Those who live on the coast have experienced this: the land temperatures rise throughout the course of the day, causing that air to rise, and the relatively cooler, high-pressure air over the water rushes inland to fill that empty space. At night, when the land temperature rises, the air over the water body is higher, so the wind direction is shifted as cooler air from the land flows back out over the water. On a much larger scale, a similar thing occurs in the tropics, where hot air is continually being pushed upwards, where it spreads to the north and south, while high-pressure air from the north and south are summarily pulled into these tropical areas, where it will start to warm up. The explanation above may suggest that wind tends to move in straight lines, from high-pressure areas to low-pressure areas in an easily predictable cycle. But it is more complex. The Earth is constantly rotating, and this affects how wind moves across the surface of the planet. The *coriolis effect* is the result of this rotational force exerted on anything moving relative to Earth. The spin of the Earth changes the direction of the wind, so wind blows towards the Equator from the northeast in the northern hemisphere, and blows from the southeast in the southern hemisphere. Directionally, they blow to the right in the northern hemisphere and to the left in the southern hemisphere. This coriolis effect bends the direction of wind across the planet, resulting in easterlies (wind blowing in from the east), westerlies (wind blowing in from the west), and adverse weather systems like thunderstorms, hurricanes, monsoons and tornadoes. How does the same movement of air that creates a gentle afternoon breeze also creates a cyclone capable of blowing houses down? When air moves across a pressure gradient (from high to low), it generates a certain amount of force. If there is a high-pressure area and a low-pressure area very close to one another, the pressure gradient will be high, the air will shift locations more forcefully, and wind speed will increase rapidly. The wind is a fascinating natural system responsible for weather patterns and atmospheric balancing across the globe. It is basically how the atmosphere moves heat around the planet. ------------