Answers to last issue's Do you Know?
1. Many modern phones need fingerprints to unlock them. What is the principle? Are fingerprints that unique?

Ans: Did you know that your fingerprints begin to form even before you were
born? In the beginning, when a fetus starts to grow, the outside layer of its skin is smooth. But after about 10 weeks, a deeper layer of skin, called the basal layer, starts growing faster than the layers above it, which makes it “buckle” and fold. The expanding lower layer ends up scrunched and bunched beneath the outside layer.
These folds eventually cause the surface layers of the skin to fold too, and by the time a fetus is 17 weeks old – about halfway through a pregnancy – its fingerprints are set. Although this folding process might sound random, the overall size and shape of fingerprints are influenced by the genes you get from your parents. So you probably share some fingerprint patterns with your family members.
But the details of your fingerprints are influenced by many other factors besides genes. For example, the shape and size of the blood vessels in your skin, how fast the different layers of skin are growing, and the chemical environment inside the womb all play a part. No two people end up with exactly the same fingerprints, even identical twins. Are we sure of this?
It was only in 2015 that a big long-term study showed that fingerprints are stable over a person’s lifetime. The ridges of a fingerprint are visible on the skin’s surface layer, but the pattern is actually “encoded” below that. Even if you have a major skin injury, your prints will come back when the outer layer heals – though you might have a scar, too. So your fingerprints are totally unique to you and have been since before you were born. No matter how much you change as you grow up, you’ll always have the set you have now, no matter how long you live.
We don't really know why people have unique fingerprints, but we have already found many uses for them! One of them is to unlock your smart phone. How does this work?

Fingerprint patterns
There are three different basic kinds of patterns on your fingers, namely Arch, Loop and Whorl, which make each fingerprint unique. What kind do you have? Compare with the pictures and find out! As you can see, these shapes are made by "ridges" of skin, with "valleys" in between.
When you place your finger on the scanner, the sensor captures the image by shining a bright light onto your finger and taking a picture using a small camera. This image is then compared with the image of your fingerprint that you first stored when you enabled the fingerprint option. In order for a fingerprint sensor to match your fingerprint with one that is stored on your phone, it needs to analyse and compare what are called "minutiae points".
Minutiae points are tiny details in the fingerprint, such as the location where ridges meet or end, or where ridges bifurcate into two branches. These minutiae points are unique to each individual and are used to create a fingerprint template that can be stored securely on the phone.
When you want to unlock your phone using the fingerprint scanner, the sensor captures another image of your finger and compares it to the stored template. If the two images match, the phone is unlocked. If they don't match, the phone remains locked. Acceptance rate i.e., how quickly and accurately it identifies fingerprint, is usually 90%, which make sure that no unauthorized person accesses the account.
Optical fingerprint sensors work by shining a bright light over your fingerprint and taking a digital photo. The light-sensitive microchip makes the digital image by looking at the ridges and valleys of the fingerprint, turning them into 1’s and 0’s, and creates the user’s own personal code (see figure).
An optical fingerprint sensor takes snapshots of your finger from different angles, so it can be fooled by presenting a digital copy of your fingerprint. A capacitance fingerprint sensor can’t be fooled easily because it uses tiny capacitor array circuits that track the detail of a fingerprint. The ridges of your fingerprint changes the charge stored in the capacitor, while the valleys (air gaps) leave the charge on the capacitor unchanged. A circuit tracks these changes in a digital fashion, and the data is compared to the stored image (see figure). The sensor only works if tiny electrical charge is transferred from the finger to the circuit and so it works only if the original human finger is placed on it. (The technique is similar to how a touch screen on a smart phone works).
Police forces are still finding new uses for fingerprints, too. As fingerprint detection and study methods have improved, detectives can even use them to see who threw a particular stone. Those little ridges can hide tiny amounts of substances too – which means they could be used to detect the use of illegal drugs like cocaine and heroin. And now forensic scientists can detect decades-old fingerprints, too – maybe allowing detectives to solve really old crimes – with a new technique that uses a color-changing chemical to map the sweat glands within your fingerprints.

Sources:
https://www.arrow.com/en/research-and-events/articles/how-fingerprint-sensors-work,
https://umbc.edu/stories/how-did-i-get-my-own-unique-set-of-fingerprints/,
Quora

2. Raindrops are falling from clouds many kilometers above us? Why don't they hurt?
Ans: There are two effects that contribute. First of all, let us assume that the raindrop is falling from a cloud that is 1 km above our heads. As it falls, due to the effect of gravity, it should accelerate, that is, move faster and faster, until it hits the ground (or your head) with tremendous velocity.
If the mass of the raindrop is 1 gm, its potential energy in the cloud is P=mxgxh, where m is its mass, g is the acceleration due to Earth's gravity (9.8 m/s2) and h is the height, 1 km. When it reaches the ground, this potential energy is converted to kinetic energy, T= mv2/2, where v is the velocity with which it hits the ground. By equating P=T, we can find this velocity, v= Sqrt(2 gh). Notice that this does not depend on the mass of the object, whether it is a raindrop or 1 kg of potatotes! This is the famous statement of Galileo, that whether you drop a feather or a stone, the final velocity of either is the same.
For 1 km height of fall, the velocity comes out to be about 140 m/s, or about 500 km/hr. Ouch! That could really hurt. Then why doesn't it? Here is where we must question Galileo's assumptions. His answer is only true when the objects are falling in vacuum. But a raindrop is falling in air, and air also provides resistance to the fall. This is called drag and acts upwards, opposite to the force of gravity. So the raindrop does not accelerate for ever. When the weight of the object equals the drag force, the acceleration becomes zero and the object then falls with a constant velocity called terminal velocity. You can easily understand this by dropping a (clean) stone in a beaker of oil or honey and compare with how it falls through water.
The drag force is complicated. The attentive reader may have already noted that the terminal velocity depends on the weight and hence the mass of the object. This is already different from our velocity calculation above. It gets even more different: the drag depends on something called the drag coefficient, which is large for large area objects. So a large flat plate will fall slower than a small ball with the same weight. And a lighter object will fall slower than a heavier object. You may have seen sky-divers falling in the "spread-eagle" position, flat, with arms and legs spread out. This makes their fall slower and more enjoyable, since their terminal velocity is then smaller. In fact, you may even have enjoyed the mist on your face, which are just suspended, tiny water droplets!
If you do this complicated calculation, the terminal velocity of rain comes out to be about 10 meters/sec for some of the largest drops of water (36 km/hr). 

Source: https://gpm.nasa.gov/resources/faq/how-fast-do-raindrops-fall, Quora

3. Why does a cyclone have an eye in the centre?
Ans: The most recognizable feature of a cyclone or hurricane is the eye. It is found at the centre, a region of calm, with low wind speeds, and low pressure, about 20-50km in diameter. The eye is the focus of the cyclone, the point about which the rest of the storm rotates and where the lowest surface pressures are found in the storm. So the eye is caused by winds rotating about the central region.
This rotation is actually caused by the rotation of the Earth, and is called the Coriolis effect. Think about this: It takes the Earth 24 hours to rotate once around its own axis. If you are standing a meter to the right of the North or South Pole, that means it would take 24 hours to move in a circle that is about six meters in circumference. That’s a speed of about quarter meter per hour.
Hop on down to the equator, though, and things are different. It still takes the Earth the same 24 hours to make a rotation, but this time we are traveling the entire circumference of the planet, which is about 40,000 km long. That means you are traveling almost 1700 km per hour just by standing there. So even though we are all on Earth, how far we are from the equator determines our forward speed. The farther we are from the equator, the slower we move. That's how objects on the same longitude stay together.
How does this change things? Suppose you are on a train on the Equator, and there is a friend travelling on a train going along, say, the Tropic of Cancer, both travelling east. Suppose you had incredibly good vision and you could see your friend waving to you! If you can see him all the time, it means that the train positions are exactly matched, one with the other. But this is only possible if your friend's train is going slower, as we saw in the example above: every point on the same longitude moves slower and slower as we approach the poles. But if you don't know your physics, you may get excited, thinking, ah! here's the chance to return that football you borrowed from your friend. So your coaches are exactly aligned, you pick up the ball, and you throw it at her straight out of the window! 
What happens to the football? You may have thrown it straight across, but any object in your train is travelling with the speed of your train, which is going faster than you friend's train! So the ball misses your friend, and hits a person in a coach nearer the front of the train (or misses the train altogether). So it appears as though the ball was deflected to the right. Of course, if you did this in the southern hemisphere, the ball will appear to deflect to the left.
Anything traveling long distances, like air currents, ocean currents pushed by air, and airplanes, will all be deflected because of the Coriolis Effect! One of the most important things the Coriolis Effect acts on are storm systems. Big storms like hurricanes and typhoons (tropical cyclones) are low-pressure systems. That means that they suck air into their center.
But as we just learned, air traveling long distances across Earth does not simply move in a straight line. Just like our soccer ball, the air being sucked into the storm deflects. This deflection is what causes tropical cyclones to spin.Storms in the Northern Hemisphere spin counterclockwise and those in the Southern Hemisphere spin clockwise. Another thing the Coriolis Effect does is make these massive storms rotate in different directions in the Northern and Southern Hemispheres.
The impact of the Coriolis effect is dependent on velocity—the velocity of Earth and the velocity of the object or fluid being deflected by the Coriolis effect. The impact of the Coriolis effect is most significant with high speeds or long distances. The Coriolis force is strongest near the poles, and absent at the Equator. Cyclones need the Coriolis force in order to circulate. For this reasons, hurricanes almost never occur in equatorial regions, and never cross the Equator itself.
You can observe the Coriolis effect without access to satellite imagery of hurricanes, however. You could observe the Coriolis effect if you and some friends sat on a rotating merry-go-round and threw or rolled a ball back and forth.
When the merry-go-round is not rotating, rolling the ball back-and-forth is simple and straightforward. While the merry-go-round is rotating, however, the ball won’t make to your friend sitting across from you without significant force. Rolled with regular effort, the ball appears to curve, or deflect, to the right.
Actually, the ball is traveling in a straight line. Another friend, standing on the ground near the merry-go-round, will be able to tell you this. You and your friends on the merry-go-round are moving out of the path of the ball while it is in the air.
Sources: NOAA SciJinks: What is the Coriolis Effect?,
NOAA Ocean Service Education: Surface Ocean Currents,
https://scijinks.gov/coriolis/.

4. Do snakes have a sense of smell?
Ans: Snakes have an amazing sense of smell. They can use their tongues to pick up on all kinds of scents in the air.
Whenever we smell something in the air, we are actually sniffing tiny building blocks called molecules. These molecules are what make up the scents of everything around us, and help us smell them.
Snakes use their tongues for collecting chemicals from the air or ground. The tongue does not have receptors to taste or smell. Instead, these receptors are in the vomeronasal, or Jacobson’s Organ, which is in the roof of the mouth. Once inside the Jacobson’s Organ, different chemicals evoke different electrical signals which are relayed to the brain.
It was once thought that the tongue delivered chemicals directly to the Jacobson’s Organ. But X-ray movies have revealed that the tongue does not move inside the closed mouth, it simply deposits the chemicals it has collected onto pads on the floor of the mouth as the mouth is closing. It is most likely that these pads deliver the sampled molecules to the entrance of the Jacobson’s Organ when the floor of the mouth is elevated to come into contact with the roof following a tongue flick.
Because it is forked, the tongue of a snake can collect chemical information from two different places at once, albeit places that are fairly close together by human standards. When snakes spread the tips of their tongues apart, the distance can be twice as wide as their head. This is important because it allows them to detect chemical gradients in the environment, which gives them a sense of direction – in other words, snakes use their forked tongues to help them smell in three dimensions. 
While the tongue does most of the smelling, snakes also use their nostrils to take in odours. We are still learning exactly how snakes use their nostrils, tongues, and Jacobson’s organs to smell the world. But we do know that some other kinds of animals use all these parts to smell, as well.
Of course, a snake’s nose is important for more than just helping with their sense of smell. Like pretty much all animals, snakes need a healthy supply of oxygen to survive. The nostrils are oxygen’s way into the body. Oxygen is really important to animals because it helps them produce fuel for their bodies.

Sources: https://askdruniverse.wsu.edu/2019/10/11/snakes-smell-tongues-noses/,
https://theconversation.com/explainer-why-do-snakes-flick-their-tongues-29935