Category Science

Why does water appear white in a waterfall though it is colour less?

When all colours get reflected from the surface of an object, it appears white. In a waterfall, water drops can be thought of as suspended in air and they may form an inhomogeneous mixture of water and air. When light enters from a lighter medium (air) to a denser medium (water) some of it is reflected (by the surface) and the rest is refracted. In a waterfall, light suffer numerous reflections and refractions by the water drops because of the refractive index difference. As a result more light gets reflected. The light refracted by a layer on top would also contribute to reflection at the next layer of drops. Thus most of the light is reflected by a waterfall. This leads to whiteness. Mist, paper, water vapour, colloidal solution, clouds, talcum, snow and white paint (there are no white pigments in white paint) and sugar also appear white because of the same reason. White paint has transparent oxides of Zn, Pb, Ti suspended in transparent solution. To see waterfall white, light should not be directional (i.e.) it should be coming from all directions. If it is directional, one would see colours as in a rainbow.

How does water stored in a mud pot remain cool?

     Water stored in a mud pot remains cool because of a phenomenon called evaporative cooling.

            Mud pots by their very nature include a number of very minute pores through which water can slowly ooze out. In effect, these pores increase the surface area of water and consequently increase evaporation. For water to evaporate it requires some energy which it takes as heat from the water itself. This results in lowering the temperature of the water in the pot. Liquid water can turn into water vapour in two ways – boiling and evaporation. Boiling takes place only at a particular temperature called boiling point, but evaporation takes place at all temperatures. While foe boiling heat is supplied from outside, for evaporation heat is taken from the water itself.

   It is for the same reason, we feel cool when a wind blows and takes away the sweat from us.

          The molecules of water or for that matter any liquid are in constant motion but all of them do not move with the same speed (kinetic energy). Their energies vary over a small range and the temperature we measure represents only the average kinetic energy of all the molecules.

            Even when left undisturbed, the fast moving molecules escape from the surface and vanish into thin air. As a result, the mean kinetic energy or the temperature of the water is lowered. This process is facilitated by a large surface area as more number of molecules comes into contact with air. (That’s why coffee in a wide saucer cools faster than when kept in a narrow cup.) The pore in mud pots provide a large surface area for evaporation and thus bring about a significant cooling of water in the pot, especially when the outside temperature is hot as in summer.

            An interesting observation could be the fact that the water in the pot can never become ice. This is because of the fact that the pitcher is not a closed system as it can also take up heat from its surroundings. Thus an equilibrium temperature is reached when the process of heat loss and gain are balanced. Further conversion of water (even at zero degrees Celsius) into ice would require a further removal of heat (80 calories/gram) to bring about a phase change (from solid to heat). This will only be possible by using another liquid as a refrigerant.

          With the same principle Double layer earthen pots are also used for preserving food and vegetables as illustrated in the labelled diagram.

                    

What causes monsoons?

           The word monsoon is used to connote a seasonal wind which flows with consistency and regularity for one part of the year and blows from the other direction for the other part of the year.

            Monsoon rains are generated over different parts of the world due to the difference in response of land and sunlight and the resulting wind movement.

            Over land, as a consequence of chemical composition and structure of the soil, only a few centimeters of the soil is heated by sunlight. But as the air has a low heat capacity, the atmosphere gets heated up rapidly.

            On the other hand, the sea is warmed up slowly because of its larger heat capacity and also because sunlight penetrates up to 200 m into the water. Hence the air above the oceans remains cooler. The temperature difference between them lies in the range of 5-10 degrees Centigrade.

            As a consequence of this, the warmer air over land begins to rise. To compensate this ascending air the denser air from the ocean has to move towards the land. (This is commonly known as sea breeze). This differential heating is the driving force for the monsoon winds.

            During the northern summer (summer in the northern hemisphere) the sun is over the Tropic of Cancer (23.5 degrees). In this period the land in the northern hemisphere gets warmed up while the southern hemisphere is cooler as it receives less sunlight.

            The land-sea contrast is much more in rectangular region shown in the figure than any other part of the world. Due to differential heating, the cooler moist air from the southern hemisphere starts moving towards the hot northern hemisphere. Once this airflow (called Somali current because it flows along the coast of Somali and Kenya region) crosses the equator the Coriollis force (a force arising due to the rotation of the Earth) turns the air to its right (in the Northern hemisphere). This flows over India and East Asia from the South West direction (hence the name south west monsoon). This moist air when aided by other factors results in abundant rain.

            The Tibet Plateau on the Himalayas acts as a potential heat source for this kind of air movement. The lifetime of south-west monsoon is about 100 days which starts early June and ends by mid September. The arrival of monsoon is a gradual process starting with a transition period when the atmospheric characteristics change from a state of extreme dryness to high humidity and slight rainfall. Thunderstorm is a feature of pre-monsoon months of April and May over North-East India.

How is depth of ocean measured?

Ultrasonic sounds are used to measure the depth of oceans and the principle behind is known as sound navigation and ranging, Sonar. Ultrasonic waves are sent from a transmitter from the surface of the sea, from a ship.

            The sound waves are reflected by the sea bottom are sensed be a receiver system and recorded. With the known value of the speed of sound in water, and the time taken by the sound to travel from the surface to the bottom and back, the depth is measured.

Why is paleontology important in the study of evolution?

Paleontology is the study of fossils. Paleontology has played a key role in developing the theory of evolution. The study of fossils has shown that the animals and plants of prehistoric times were very different indeed from the animals and plants of today. By reassembling fossils bones, scientists can get an idea of the size and shape of the creatures that inhabited the Earth before the appearance of humans. Teeth and shells that have been fossilized also tell us much about the lifestyle of these creatures. Fossils of insects are often found embedded in amber, which is a transparent rock nodule formed when the resin from trees petrifies. Small creatures are sometimes trapped inside the amber, and are perfectly preserved to give us a glimpse of ancient insects.Trace fossils are also important in the study of evolution. Trace fossils are tunnels, worm casts and tracks that animals have made on soft sediment that has later hardened into rock. These imprints of animals on the move reveal much about the lives of ancient species, including their size, gait, stride length, and speed.

Why have fossils given rise to many myths?

Fossils are the remains of animals or plants which once lived on the Earth millions of years ago. When fossils were first discovered, people did not really understand what they were, and so, many myths and folklore developed over time, about where fossils came from.

            The coiled shell of some fossils led the Ancient Greeks to believe they were related to the ram-god Ammon. In China, these were also thought to resemble horns, and were called ‘horn stones’. In England, they were believed to be coiled snakes that had somehow lost their heads, and turned to stone. Echinoids, also known as sea urchins, are amongst the most commonly found fossils. In some places, they are considered to be snakes’ eggs, which have the power to protect one from deadly poisons.

Fire Eating Dragons

            Over 40,000 years ago, giant bears roamed over Europe. Some of them were buries in caves while hibernating, and over hundreds of years, their bodies turned into fossils with huge canine teeth. When these fossils were discovered in the Middle Ages, they were thought to be the remains of a mythical creature, the fire breathing dragon.