Saturday, February 2, 2013


Why is rainbow seen after rain?
After a rainfall, the clouds break and lie hanging in the sky and the sun is also visible. The Sun’s rays, falling on the water drops, are dispersed, totally reflected at the back of the drops and then again refracted into the eye of the observer with his back towards the sun. These dispersed rays which have
suffered deviation give rise to a concentrated beam of light indicating all the colours of light.

Ice wrapped in a blanket or packed with saw dust does not melt away quickly. Why?
Ice wrapped in blanket does not melt, because blanket being bad conductor, cut off heat rays. Saw
dust is also a bad conductor of heat. It protects the ice from the external heat and prevents it from
melting away.

Petrol fire cannot be put with wamter. Why?
Petrol has less density than water. So when water is poured the petrol floats on it and keeps on burning. The temperature of the burning petrol is so high than the water and it would be evaporated before it can extinguish the fire.

Fuse wires are always provided in electrical installations.?
Fuse wire is a high resistance wire and when it is put in the series of electrical installations under strong
currents, the wire melts away avoiding damage to main installations.

Copper wire cannot be used as heating element in electric heaters.?
Copper melts at 1083°C and also reacts with air to form a black powder.

Glass when heated cracks while metal does not.?
Glass is a bad conductor of heat. On heating, only the upper layers of the glass expand while lower ones remain unaffected, hence it cracks due to unequal expansion. Metal is a good conductor of heat and so all its bulk gets uniformly heated up. Thus the expansion also has uniform surface strain, hence no cracking.

Why a cyclist has to apply a greater force at the start than when the cycle is in motion?
A cyclist has to apply a greater force at the start in order to produce momentum, but once a suitable
momentum has been gained only then a smaller force is required to balance the frictional forces.

Why sea-water is saline?
Because the rivers falling into the sea, bring deposits of salt along with them during their journey from mountains and plains and this process has been going on for ages.

The sky appears blue, Give reasons, why?
The blue colour of the sky is due to the scattering of light by dust particles or air molecules. This scattering is inversely proportional to the fourth power of wavelength. Consequently the shorter wavelengths are profusely scattered and when we look at a portion of the sky away from sun, we receive this scattered and re-scattered light which is rich in blue (short wavelength).

An electric bulb makes a bang when it is broken?
The bulb has a partial vaccum. When it is broken the air rushes in to fill the vaccum and thus a noise is

How is it that food gets cooked quicker in a pressure cooker than in an ordinary vessel?
In a pressure cooker, on account of the increase of pressure, the boiling point is raised, so the food gets cooked quicker than in an ordinary vessel. A pressure cooker is very useful in high mountains where, on account of low pressure and hence low boiling point, the vegetables cannot be cooked properly.

A burning candle gets extinguished when covered with a tumbler. Why?
The tumbler cuts off the oxygen supply which is necessary for its burning.

Why rain water is soft but river water is hard?
River water during its course in the mountainous terrain dissolve the calcareous matter or lime
compounds and thus becomes hard. While during evaporation only the molecules of water go up and
minerals are left behind; hence the rain water is soft.

Why is water from hand pump warm in winter and cold in summer?
In winter, the temperature outside is low and the unexposed water underground is at a higher
temperature. In summer, the outside temperture is higher than the temperature inside. Hence water is
warm in winter and cool in summer.

An iron nail floats on mercury but sinks in water. Give reasons, why?
An iron nail floats on mercury, because the weight of mercury displaced by it and hence the upward thrust is more than its weight, the density of mercury and iron being respectively 13.6 gm per c.c. and 7.6 gm. per c.c. The density of water being 1 gm./c.c., the upward thrust is much less and hence the iron nail sinks.

A flash of lightning is seen before the sound of thunder heard. Why?
It is because of the fact that light travels faster than sound.

One leans forward while climbing a hill. ?
While leaning forward, the centre of gravity of the body also shifts forward and this helps climbing.

A dead body floats in water after some time.?
A body weighs less when weighed in water because of the apparent loss in weight being equal to the weight of the water displaced. There is an upward thrust exerted upon a body immersed in a fluid called buoyancy, equal to the weight of the fluid displaced and thus the dead body floats in water after some time.

Why are we advised to empty the ink from our fountain pen before going up in an aeroplane?
As we go up in an aeroplane the air becomes rarer and the pressure of the atmosphere therefore falls, so that the volume of the air inside the fountain pen will also increase and the ink will be pushed out thus spoiling the clothes and hence the advice.

Wet clothes dry slowly on a rainy day. Why?
On a rainy day the air in the atmosphere contains more water vapour than on a dry day. As a result evaporation is slower.

Why it does not hurt when we cut our nails?
Nails are the parts of the body which have no connection with either the blood vessels or the cartilage and hence the nerve system is unaffected. Consequently cutting them will not injure us.

It takes more time to cook meat and vegetables at hill stations. Give reasons, why?
Meat and vegetables can be cooked properly and quickly at a temperature of 100°C. Since at hill stations, the pressure is decreased and hence the boiling point is lowered thus causing a serious drawback in cooking which will therefore take a much longer time.

A blotting paper absorbs ink. Why?
A blotting paper contains minute pores. These pores will be filled with ink. Thus the ink will be retained by the paper.

How a ball which falls down, bounces up?
A ball on falling to the ground is slightly deformed. On account of the elastic force coming into existence due to the deformation the ball tries to recover its original size. In doing so it presses the ground and in accordance with Newton’s third law of motion it receives a reaction upwards and hence it bounces up.

Why a needle sinks in water, whereas an iron ship floats on it?
The specific gravity of a needle which is a solid piece of steel, is decidedly greater than water and it, therefore, sinks in water; whereas an iron ship is so designed that the total weight of water displaced by it is greater than the weight of the ship itself. The ship, therefore, floats by the upward thrust of water.

Why a convex mirror is used by the motorist to see the road behind him?
Due to the formation of miniature size images, a convex mirror has a large field of view. Consequently, the motorist sees a large number of objects behind the car simultaneously. It also avoids reflection of the sun rays which are diverged and scattered.

Why is it dangerous to allow extra passengers on the upper deck of a double-decker bus?
There are chances of the double-decker bus tilting, if the upper deck gets over-loaded with passengers. In fact, the upper deck and lower deck act as a counter-balance to each other and secondly, the upper deck has no support on top of it. With extra load on the upper deck, the centre of gravity of the bus is raised and the resulting instability can make the vehicle tilt.

A hydrogen balloon rises. Why?
Hydrogen is lighter than air. The weight of hydrogen in the balloon is less than the weight of the air displaced by it.

Why an iron nail gains weight on rusting?
Rusting is nothing but iron oxide. If fact, iron in the presence of moisture absorbs oxygen to form iron oxide. Hence on absorption, iron gains a weight equal to the amount of oxygen consumed.

Give scientific reasons as to why a clinical thermometer should not be dipped in boiling water?
A clinical thermometer is an ordinary Fahrenheit thermometer, calibrated from 95°F to 110°F, whereas boiling water usually acquires a temperature of more than 100°C or 212°F in water. It is, therefore dangerous to dip a clinical thermometer in boiling water because it is difficult for it to accommodate such high temperature, as a result of which it might burst.

Why is it more difficult to breathe on mountains than on plains?
On the mountains the density of air is much less than in the plains and so the oxygen content for volume is reduced considerably. During the act of breathing a definite supply of oxygen is needed per breathe which being deficient, breathing becomes difficult.

Why cloudly nights are warmer than clear nights?
Cloudy nights are warmer than clear nights because clouds prevent radiation of heat from the ground and air.

A green leaf appears green in daylight but looks dark in red light. Why is it so?
During daylight, a green leaf appears green because out of all the constituents of white light, it reflects green only, all other colours being absorbed. But when red light falls on it, it is all absorbed and hence the green leaf reflects none and looks dark.

Why the outer surface of a glass containing iced water becomes wet?
A tumbler containing cold water cools the air near it so that a film of moisture is deposited on the outside of the tumbler by the condensation of water vapours present in the air.

A solar eclipse can occur only on a new moon day. Why?
Soloar eclipse is due to the full moon coming in between the sun and the earth, and hence it is natural that it should be a new moon day.

Why the lunar eclipse occurs only at full moon but not every full moon?
The lunar eclipse occurs only when the moon is full and is near to one of its nodes which does not occur every full moon.

Where days and nights are equal throughout the year and why?
Day and nights are equal throughout the year at the Equator. During the course of the earth’s revolution round the sun, one half of the Equator is always in the light and the other half is in the dark. i.e., the circle of illumination always cuts the Equator into equal parts.

What is the difference between a star and a planet?
Star is the name given to a fixed celestial body which has its own light whereas Planet is the name given to a celestial body which revolves round the sun in elliptical (regular oval shape) orbit. A planet has no light of its own but it reflects light of the sun.

Where days and nights are longer in the year?
Longest day : 21st June. Longest night 22nd December (in Northern hemisphere).

Why are mountains cooler than plains?
Because (i) the air of the mountains is rarer than that of the plains and contains fewer dust particles. The air on the mountains absorbs less heat than the air on the plains, (ii) the heat absorbed during the day on the mountains radiates very quickly at night owing to the rarity of the air and nights on the mountains are cooler, (iii) due to uneven surface of the mountains, the major portion always remains in the shade. The sun does not heat much of the land which may heat the air.