Answers to last issue's Do You Know?

1. We send up so many satellites into space. Can they collide?
Ans: There are actually hundreds of thousands of objects in space. In fact, there are an estimated half a million pieces of what is called space debris, particularly in low Earth orbits. These are not the communication satellites that we use for broadcast or for mobile phone calls around the world, but the sort of satellites that observe the Earth. For instance, the International Space Station is in this particular orbit. 
Now, these half a million objects vary from little flecks of paint to big objects. Even these flecks of paint can cause severe damage: imagine things going around the Earth at almost 8 meters per second. The biggest of these is about 20,000 pieces of space debris and this is tracked carefully using radar. It sounds like a lot but the Earth is pretty big. Of course we do not want to hit one of these when we are launching a satellite. So, we launch into orbits where there are no stuff. For this it is important to track the debris continuously. We do not know where tiny bits like flecks of paint are. But then the probability of hitting a fleck of paint is low. 
So, when we are planning an orbit for satellite, it is basically about how fast and exactly the sort of the angle we shoot it up over the Earth at. Coming to telecommunication satellites, they sit in what are called geostationary orbits. Essentially, they orbit the Earth at the same speed as the Earth rotates. They sit above the same part of the Earth all the time. These satellites actually have to navigate their way through the space debris without hitting the low-Earth satellites that are orbiting. This is actually planned and sometimes they have to divert their course. Imagine crossing the road, when you have to weave between the traffic, this is what they do to get up to their high orbit. These are all things taken into account, while planning a launch.

2. We usually feel the forehead or neck of a person to check for fever. Does fever raise temperature equally over the entire body?
Ans: Let us first ask why we run a temperature at all. We get fever because some chemical is being produced in our body that triggers the brain's temperature regulating centre called hypothalamus. It says, turn up the thermostat.
Why does this happen? Usually, this happens because of an an infection. When microorganisms cause inflammation, they damage tissues. They release various inflammatory chemicals and also, particles from the surfaces of the microorganism. The bacteria for instance have chemicals called lipopolysaccharide, LPS, which is part of the wall of the bacterium. This triggers the hypothalamus to sense that there is an infection and it turns up the set point for body temperature. This triggers the release of more thyroxin which increases metabolic rate and that makes one hotter. It also triggers release of adrenalin which increases metabolic rate and that makes one hotter, it causes blood to move more towards the centre of the body, slowing down the rate at which one loses heat. This puts your temperature up. 
There are other reasons why one might run a temperature, but those are the most common reasons. This means that the core body temperature will go up and usually, it can go up from a normal of about 37 degrees C to perhaps 40. The highest ever recorded temperature in a human being actually was 45 degrees and they survived. But most people cannot go much beyond 40. The body does this because when you increase temperature, you make it harder for infecting microorganisms to grow. You make it easier for your white blood cells to move around, and you therefore frustrate the bug and benefit the body. So, it is a kind of defensive mechanism. 
This cannot happen equally everywhere for the simple reason that one is shunting blood towards the core so the core will be warmer, but the peripheries may actually be shut down and be cooler. There will be a global increase in body temperature right across the body when one is running a temperature. So, if you put your hand on that person's forehead, yes, it is going to feel warmer.
Best measurements are made where there are body cavities close to major blood vessels. Such places are the mouth, rectum, under-arm and ear-drum. The forehead is often used because it is the least invasive. But the best way of measuring temperature is to get the core temperature; for this using the thermometer is a must. For this, oral or under-arm measurements are preferred although the rectal measurement is most accurate.

3. Sometimes rain has a certain smell and sometimes it does not. What is it that is giving the rain its smell?
Ans: Rainsmell is called petrichor because petros in Greek means rocky and chor means smells. Nobody really knows why this happens. In the 1970's, a scientist named Nancy Gerber did some chemical experiments and made some measurements. 
Gerber found that there were several chemicals, some of which coincide with a soil microbe called actinomycete. These filamentous bugs live in the soil and grow throughout the soil. They break down various detritus and a product of their metabolism are these chemicals.
When the raindrops come down, they splat into the ground and they elevate particles of these microorganisms, their by-products and their metabolic waste into the air and it happens to have this kind of smell. One name given to it is geosmin after one of the chemicals. Scientists believe that as the drop hits the ground, it compresses and captures a little bit of matter and air (some bubbles) as it lands. This then has to come bubbling up through the liquid. As it does so, it carries with it some of these particles from the soil. Like from an aerosol spray they get distributed out into the air. Then you smell them. 
This does not happen all the time, so presumably, for the smell the conditions need to have just the right weight of rain and the right kind of soil. If you have had relentless rain day after day, it has damped down all the particles probably anyway. They have all been washed away or you are already used to the smell. Thus if you had a long pronounced dry spell, lots of the chemical is in dry soil, easy to release into the air. As rain comes down, they splat up into the air, you breath it and then you really notice. That is what geosmin is supposed to be.

4. When we make ice cubes at home they are always white, but I have seen clear ice cubes in restaurants. How can we make clear ice cubes?
Ans: First of all, why is ice not clear? Snow is ice but looks white, whereas ice on a pond is transparent. Why this difference?
Well, snow particles are lots of tiny ice crystals. If you have lots of tiny crystals, when light goes into the crystals then it gets reflected and it gets bounced about all over the place and all of the different colours of light come back towards you. That is why it looks white because when you mix all those colours together, you get white light.
So, when you have an ice cube that is not clear, it is because there are lots of little, either fractures in the ice cube or more commonly, lots of little crystals that have all formed to produce one giant ice cube.
How then can you end up with a single ice cube that is a single crystal which is what you need in order to not disrupt the path of the light? You need to do what is called nucleation, the formation of the ice cube from one position only.
In fact, when you are making a jet engine, you need to do exactly the same thing because when you make the metal parts of the engine, the strength is in growing your engine parts as one single metal crystal. So, you pour molten metal into a mould and then you drop in, or you initiate, one tiny form of crystal into one place and the whole thing then crystallises following the same crystal structure.
So, in an ice making machine, usually, there is a stainless steel tube which will conduct heat really well. First, you make that cold. So there is a central core which is also cooled and this way, there is a lot of surface area in contact with ice. This makes the temperature drop really fast, so the freezing process starts around that central cooling probe. The crystal will then grow out from there towards the margins. Therefore, you are more likely to have a single crystal, and hence more likely to have nice clear ice.
This is one reason why such ice cubes are circular shaped, with a hole in the middle. Because that way, you have got as much contact of the metal that can take the energy away from the water, as possible. This drops the temperature quickly and you make ice cubes fast. The by-product is that you generally get them starting the freezing process in one place. They nucleate from one place, you get one crystal, nice clear ice.
For the same reason, if you happen to drop and break a clear water-glass so it shatters, you will get white fragments, not bits of clear glass.

5. Can one sneeze in sleep?
Ans: There is no reason why you should not sneeze in your sleep.
What happens when we sneeze? You have a circuit in your brain which detects movements or irritation of the lining in your nose. There are many tiny hairs there which if tickled or plucked, strongly stimulate this reflex. These signals go back to the brain stem which connects the spinal cord to the top part of the brain. This is where we have the centres which control breathing, coughing, blinking, the amount of saliva and eye secretions and tears. Sneezing and hiccupping is coordinated there as well.
When we’re awake, particles, allergies or an illness can stimulate the nerve cells in the nose. The nerves then send signals to the brain in order to initiate a sneeze to get rid of whatever is irritating it.
But during a certain stage of sleep called REM (rapid eye movement) sleep, certain neurotransmitters shut down---this is called REM atonia. This means that the motor neurons are not being stimulated, so they aren’t sending these signals to the brain.
It is possible sometimes that you receive enough stimulation to sneeze while you are asleep. But the amount required would wake you up long before you could actually sneeze!
Of course, if you are in very deep sleep, there is no reason why you should not sneeze, though you may not remember doing it when you awaken.

6. Why does petrol create a rainbow on water?
Ans: When you see an oil film on the road on a rainy day, it gives rise to bands of beautiful colours. Small amounts of oil are usually present on the road surface (for instance, lubricating oil from cars). When it rains, drops of oil float on the layer of water that collects on the road because the density of oil is less than that of the water. This is the same reason that wood floats on water. Commercial oil formulations usually contain an additive that causes the oil drops to spread out into a thin film atop the water. That film is thickest in the center of the patch, or oil slick, and thinnest at the periphery.
Both the top and bottom surfaces of this oil film can reflect light. If the path difference between two light rays is a multiple of the wavelength, there will be what is called constructive interference.  A light ray will pass through different thickness of oil when the angle of reflection varies. The wavelength corresponding to the constructive interference also differs and this causes the reflected light to have various colours. As a result, a rainbow-like colour pattern is shown on the oil surface and this phenomenon is called thin-film interference. 
In the figure, the interference between the two outgoing light rays that have been reflected from the top and bottom of the oil/petrol film causes the effect.

Sources: Science Forum, Cambridge University; Scientific American; Physics World