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 water. 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.
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.
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 produced.
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 temperature 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.
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