Answers to last issue's Do You Know?

1. Why do breaking waves at Marina Beach (in Chennai) look white?
Ans: There are two questions here, really. Where does the white in breaking waves come from? And why does this not happen in calm water?
The crest of a breaking wave contains lots of tiny droplets and bubbles. They are all transparent individually, but together they act like something opaque and white. Light scatters in many directions as it passes from air through a single droplet of water. Individually, each droplet scatters the surrounding light just a little bit, but collectively, the droplets scatter a great deal. This scattering mixes together all the different colours of light from the surroundings, blending them into white light.
Thus, coloured light bounces in and around tiny bubbles in the water which mix together to look white. So, why do we see this effect in breaking waves, but not in calm water?
Here it is important to note that light scatters when it passes between different materials such as water and air. The water in the ocean meets the air only at one surface, but the droplets in the surf meet millions of surfaces. So, they scatter the light more in a small area. That is why only the part of the wave that has broken apart appears white.
Scattering makes the crest of the wave look white with light from the surroundings. In some places though, there may also be light coming from inside the water. The crashing motion of the waves can agitate bioluminescent microorganisms. At night, you might see them glowing blue green in the surf. 

2. What effect does physical exercise have on the brain?
Ans: We all know that physical exercise has a great effect on one's mood. So clearly it does affect the brain, but are the effects?
It is known that exercise stimulates endorphin release. These are chemicals and when they are released, they bind to receptors in the brain. The subreceptors that they bind to are involved in pain signalling and they actually disrupt pain signals that you get. Thus, they reduce some of the feelings of pain that many of us get when we exercise hard. 
It is easy to see that exercise increases oxygen supply to the brain and this can be quite helpful to keep nerve cells functioning optimally. But scientists also speak of implications in neurogenesis which is the ability to actually grow new neurons inside the brain, in the hippocampus. This is the region heavily involved in learning and memory. 
Research shows more. There is a stress hormone called cortisol which is released in times of extreme stress. Apparently, exercise increases something called brain derived neurotropic factor, BDNF, which helps protect against the cortisol stress effect and thus makes us resilient to stress. 
Thus physical exercise has many positive effects on the brain, can help elevate your mood when you are feeling low. What is very difficult is to persuade someone feeling low to go do strenous exercise!

3. Why do healing wounds itch?
Ans: Actually only recently have we found that there are special nerves in the skin for this, actually itch specific nerve fibres in the skin. Their only job is to signal to the spinal cord that an area of the skin is being irritated in a way that we would perceive as itchy.
These nerve cells are activated in a number of ways. One of them is mechanical. For example, if you have an insect crawling on you, the crawl triggers these nerve cells. Another way is chemical. There are certain chemicals that irritate the skin, and again trigger these cells.
Now when you have a wound, the wound closes by cells around the margins of the wound, proliferating, in other words, growing. They then migrate from the margins of the wound, down into the base of the wound. It turns out that the inside of the wound is at a different voltage than the margin, causing an electrical gradient. The cells flow down this electrical gradient, so they know where the base of the wound is. They then unite with their cellular counterparts, and stitch themselves into place. Then they start to contract, contractile filaments which pull the wound closed.
As they do this stitching, they elicit a mechanical stress which the itch sensitive nerves respond to. At the same time, there are chemicals which get released in a healing wound. These chemicals help in healing, but also promote the activity of the itch sensitive nerves.
Therefore, a wound that is closing up feels itchy for mechanical and chemical reasons -- precisely the same reasons why those nerve cells get stimulated in the first place.

4. Can we burn off fat by solving a hard mathematical problem instead of doing hard physical exercise?
Ans: The answer is a definite NO, but how are we so sure?
Scientists wanted to measure oxygen consumption of the human brain, and test whether the brain was like a muscle - that if you worked it harder, it had to use more energy. So, they got people to sit down and solve difficult mathematical problems and measured the energy consumption of their brains. They found that there was no detectable change.
So, doing difficult math problems will certainly not enable you to lose weight. The best thing you can use your brain for is to design a good exercise regime and a diet. 

5. Do all thunderstorms have lightning?
Ans: By definition, if you have a thunderstorm, you should be hearing thunder. That means you are hearing a shockwave. This means that something has heated a patch of the air to a very high temperature. This has made the air expand supersonically, making a shockwave that then comes towards you through the air as a rippling roll of thunder. Now you cannot really have such heating of the air without some kind of an electrical discharge which is what we call lightning. 
In fact, when people have done calculations and measurements, a lightning bolt actually registers a temperature of about 30,000 degrees C which is 5 or 6 times hotter than the surface of the Sun.
So if you have a thunderstorm without a lightning bolt it is probably just that the lightning is masked behind a layer of cloud (or something else) and you just did not see it.

6. How much do space missions cost? Are they worth that much?
Ans: Recently, there has been a great deal of discussion in India on the cost of Mangalyaan. People have raised the question whether a poor country can afford such an expense.
In actuality, the cost of Mangalyaan is calculated to be less than 500 crore rupees which comes to less than 5 rupees per person in the country. But then it is hard for us to comprehend that kind of money. As a comparison, the amount of money spent on crackers on Diwali day in India exceeds the cost of Mangalyaan. We should also note that it is about a tenth of the Mars expeditions carried out by other countries.
As a comparison, the Cassini-Huygens cost about 4.5 billion US dollars. But even this was over 15 years. The Mars Science Laboratory costs NASA about 2.4 billion US dollars, spent over about 10 years over 7000 scientists and engineers. NASA claims that Americans spend more money on chewing gum than they do on NASA! 
The most important thing to note is that ISRO's money is not being spent in Mars or on the moon. All of it is spent on Earth, most of it in India. It is spent mostly in high-tech industry, paying people and paying industry to develop technology. The benefits of developing such technology reach all of society. Moreover training generations of scientists and engineers in such highly skilled jobs leads to human resource generation, often leading to tremendous breakthroughs in ideas and innovation (which would not have been possible without such training). So it is not only money well spent, but in terms of nation building, only a small amount of money.

Source: Science Forum, Cambridge University