2. Why does red poppy look red?
It is because it reflects only the red-colour, and absorbs the rest of six colours.
3. White light passing through a glass prism gives rise to a coloured pattern on the wall. Why?
White light consists of seven colours: violet, indigo, blue, green, yellow, orange and red. Each coloured light has its own refractive index — violet maximum red least. All colours on meeting the surface of the prism get refracted or bent: violet the most and red the least. Thus white light completely separated into its constituent colours emerges out as a coloured patch on the wall, known as the spectrum.
4. How does a bulb emit light?
The current passes through a tungsten wire of high resistance, which becomes white hot, and thus it begins to emit light.
5. Are two blankets warmer than one of double the thickness of each; if so why?
Yes, two blankets are warmer because the air additionally closed between the two does not allow that heat to go out.
6. Why are lightning conductors fixed at the top of high buildings?
To protect the building from the destructive effect of lightning, when a discharge takes place from the cloud, the lightning conductor provides it an easy conducting path to the earth via the outside of the building, without damaging it.
7. Why are two eyes more valuable than one?
We can view the object from two points at once, giving us an idea of solidity (depth) also, and not of flatness (height and width) only.”
8. A boatman pushes the bank with his pole, Why?
The bank will push the boat away from it. Action and reaction being equal and opposite.
9. A hydrogen balloon rises. Why?
Its weight is less than the weight of the air displaced. The net upward thrust on the balloon carries it up.
10. You lean forward while climbing a hill. Why?
This enables us to keep the line passing through the centre of gravity vertically downward, within our feet, thereby keeping the body in stable equilibrium.
11. Why do you see rainbow after rain?
The water-drops suspended in the air act as a prism and disperse light into seven colours.
12. Wet clothes usually dry more quickly on a warm day than on a cold day. Why?
On a warm day, the air carries less moisture than its point of saturation and thus it can absorb moisture from the wet clothes more rapidly. Also the high temperature of the air helps faster evaporation.”
13. When a piece of stone gets hot in the fire, pieces tend to fly off from its. Why?
This is due to the outer layers expanding before the inner part becomes hot, so that they break away. Similarly, surface rocks break up in regions, where there is a great difference between day and night temperature — thus assisting in the formation of soil or sand.
14. A metal tea-pot has an ebony handle. Why?
Ebony is a bad conductor, so it does not carry heat from the tea pot to our hands.
15. Why does a coin lying on the bottom of a tall cylinder full of water seem to be raised?
Rays starting from the coin bend outwards due to refraction at the water-air interface. Thus, as they move out into the air, they appear to be coming from a place higher than the actual place of coin.
16. Why do diamonds shine at night?
The rays of light entering it get internally reflected from most of the faces, on account of high refractive index so that from the few faces that permit light to get out, a lot of light comes out.
17. Why do the front wheels of a motor car usually lean outward slightly?
The front wheels of a motor car slightly lean outwards to allow for difference in angles, when turning round the corner to give the car greater stability.
18. How does a flowering plant reproduce itself?
It consists in the fertilisation of ovaries by pollens, which are carried by insects or wind from one plant to another. The fertilised ovaries become seeds, which when sown, reproduce the plant.
19. Why is one’s breath visible in cold, but not in hot weather?
In cold weather the water vapours contained in the exhaled breath condense into small droplets of water, which become visible.
20. How does a thermos or Dewar’s flask keeps a hot liquid hot and cold liquid cold?
The thermos flask is a double-walled bottle in which the inner surface of the outer wall and the outer surface of the inner wall are silvered to prevent radiation of heat. The vacuum enclosed between the two chambers does not allow the inside heat to escape either by conduction or convection. Thus, heat transfer through all three modes, radiation, conduction or convection is minimised.”
21. Why is a compass used as indicator of direction?
The magnetic needle of a compass under the influence of earth’s magnetic field always lies in a nearly north-south direction. Hence we can detect the direction.
22. Why in a country like Ladakh, it may be very hot in sun-shine, but bitterly cold in shade?
Places at a great height like Ladakh have a rarefied atmosphere, which offers little obstacle to the rays of the sun which are, therefore, scorching. But the rarefied air absorbs little heat from the rays of the sun, therefore, the atmosphere remains cold. Hence it is very cold in the shade.
23. Why does a tennis ball bounce higher in Shimla or Darjeeling than it does in Kolkata?
Shimla and Darjeeling being at a higher altitude are farther away from earth’s centre than Kolkata, so the downward pull due to gravity is less in Shimla or Darjeeling. Hence the acceleration due to gravity ‘g’ is less there than in Kolkata. Besides, the air is rarefied, therefore it offers less resistance, Hence the ball bounces higher.
24. What would you do if you see a man wearing nothing but a loin-cloth in contact with a live electric wire?
Catch hold of him by the loin-cloth and pull him off the live electric wire. Cloth being a bad conductor, will not let the electricity pass.
25. Why does an electricity bulb make a bang when it is broken?
An electric light bulb has a partial vacuum. When it is broken the air rushes in to fill the vacuum, thus causing a bang. This is called implosion.