Do You Know? 1. When you see a jet plane flying high in the sky, there is a long white line of clouds like a track behind the jet. What causes this? These clouds are called condensation trails or contrails for short. Some days, the contrails will form thin lines that cross the entire sky. Other days they will be much shorter, and on some days there will be none at all. The jet engine burns fuel. The main products of such hydrocarbon fuel combustion are carbon dioxide and water vapour. At high altitudes this water vapour emerges into a cold environment, and the additional water vapour can push the water content of the air past saturation point. The vapour then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the vapour trail or contrails. The vapour needs an energy drop (and therefore, time and distance) before it condenses. Thus the contrail forms some way behind the aircraft's engines. On a clear day, this "gap" between the jet and its contrail can be clearly seen. Also, the water vapour requires a trigger to encourage condensation. The exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapour to rapidly turn to ice crystals. Exhaust vapour trails or contrails usually occur at above 26,000 feet. where the temperature is below -40°C. Saturation point and condensation You may have noticed that on certain days in the winter, your breath will form a cloud of condensation when you exhale. In the summer, however, you don't see your breath. Cold air can hold a lot less moisture than warm air, so in the winter, when the moisture in your breath hits the cold air, the moisture condenses into a visible cloud. The same thing happens when a jet engine "exhales." If the temperature, winds and humidity in the upper atmosphere are right, long, white contrails form when the moisture in the exhaust condenses. 2. What would happen if you drill a hole through the center of the Earth? Nothing much! Things get more interesting if you try and throw something down the hole, perhaps even yourself. You know that you are held on Earth by its gravity. Any object has a weight on Earth's surface depending on its mass (mass times acceleration due to gravity is its weight). As the object falls into the tunnel, more and more of the Earth's mass is around or outside you, so the net force of gravity keeps on decreasing. This means that your weight keeps decreasing as you fall in. At the centre of the Earth, there is no force of gravity, and so no force pulling (or pushing) you any where. In some sense, the force due to equal amounts of Earth around you in all directions is cancelling out and you are weightless! Before weight watchers start rushing to jump in and lose weight painlessly, beware! If you are falling in to the centre of the Earth, it is true that the force on you keeps decreasing, going to zero at the exact centre of the Earth. But there is still inertia. At the Earth's centre, there is no force on you due to gravity, but you would have reached there at a very high speed. This will cause you to overshoot, and so you will continue to go through the tunnel, past the centre. Once you pass the centre, you will start to slow down, and acquire weight. Just like a pendulum goes to and fro, you will come to a complete halt only on the other side, at the surface of the Earth (and with all your original weight intact!). If there is no friction, you will simply keep oscillating for ever. Or until a kind friend helped you out. 3. Why does smoke come from a fire? Since we are not used to lighting fires and watching them burn, it may be easier to understand this discussion by watching what happens to an incense-stick (agarbatti or oodhubathi). When the fire is lit, it produces a lot of heat but there is essentially no smoke. Those of you who have seen a wood or charcoal fire will have noticed that it does not smoke when there are only embers glowing (which is the right time to put in that brinjal to roast!) Once you add something to this fire, it again starts to smoke. Most objects such as wood, incense-sticks, vegetables or food, contain water and hydrocarbons. Hydrocarbons like cellulose and others are volatile. This means that they evaporate when heated. So, when you add that piece of wood, it starts to heat up and the hydrocarbons evaporate. This is seen as smoke. Once the wood really heats up (or as is the case with embers), the temperature is high enough that the hydrocarbons don't evaporate but actually burn (and as their name suggests, they burn into carbon-dioxide and water vapour). Once this happens, there is more heat but no more smoke. It is the other way around with an incense-stick. When it is lighted with a match, it is very hot and the hydrocarbons it contains also burn, so that there is no smoke. When the flame is put out, it begins to cool so that the hydrocarbons stop burning and merely evaporate, thus causing the familiar smoke you see. The various fragrances used determine the smell of the smoke and the central bamboo stick ensures a continuous supply of heat. The non-burnable parts of the stick fall off as ash. 4. At night, it takes several minutes for the eyes to get used to darkness. Why is this? There are two main kinds of cells in the eye: rods and cones. Cones are responsible for colour vision and work best in bright light. Rods see black and white images and also help in night vision since they are more sensitive than cones in dim light. All light is emitted and absorbed in units called photons. Rhodopsin is a chemical that is found in the rods in the eye. Absorption of photons actually is the absorption of energy, which is used to break up the rhodopsin into a retinal and opsin molecule. This break-up is interpreted by the brain cells in terms of the act of seeing. For light sensitivity, the split molecule has to recombine so that it is again ready to be split up by the incoming light. This recombination takes place at a fixed slow rate. The picture shows the structure of rhodopsin. The bunch of spiral shapes in the centre of the lipid or fat "fence" at the top is rhodopsin. The knotted spiral shapes hanging below are transducin which is found in the disc membrane of rods and cones. Transducin is responsible for converting the light signal to a nervous impulse. So if you have been in a room with a lot of light, chances are high that a lot of rhodopsin has been broken up. If you suddenly switch off the light, there is no rhodopsin so the rods are not able to "see" at all. Now the cones are still active, but they need a lot of light to work so effectively you cannot see at all. After a while when the rhodopsin has recombined, your vision returns. Do you know that the Nobel Laureate C.V. Raman worked extensively on vision? He was among the first to prove that the eye has different sensitivity to different light and that the eye is most sensitive to the green colour in the dark. He also showed that a certain minimum light is required to see colour at all (all you see in the dark are grey shades). The retinal used in the eye is derived from vitamin A. If you don't eat enough of this, there is not enough retinal in the rods and therefore not enough rhodopsin. People who lack vitamin A often suffer from night blindness -- they cannot see in the dark. 5. Most glasses have an anti-reflective coating. Why? This is to avoid what is known as back-glare when light from behind hits the inside of your glasses and enters your eye. Since the inner or back surface of a lens is usually concave, the effect is like a magnified mirror image and can temporarily blind you. Sometimes, you can see a person on TV whose glasses are reflecting the camera in front of him! This means he is not wearing glasses with anti reflective coating. This can be important in night-driving when a vehicle is trying to overtake you from behind: you can be blinded by that vehicle's head-lights. To avoid this, an anti-reflective coating made of a very hard thin film (for example, of magnesium hexafluoride) is coated on the inside surface of the lens. This reduces the reflections caused by light entering from behind the wearer. Front glare from highly reflective surfaces such as metal and glass, or from the head-lights of an on-coming car can be reduced by using polarised lenses.