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MAY 04, 2014

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Q. 22. Why does milk look white to us?

A. It reflects all the colours in the spectrum, without absorbing any of them.

Q.23. Why does the white light passing through a prism produce a coloured pattern?

A. White light contains seven colours: Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR). Each coloured light has its own refractive index, with violet having the maximum and red the least. All colours on striking the prism get refracted or bent: Violet the most and red the least. Thus white light get split into its component colours, known as the spectrum.

Q. 24. Why does an ordinary bulb emit light?

A. The current, passing through the tungsten wire of high resistance, makes it white hot, and thus it begins emitting light.


Q. 25. Two blankets are warmer than one blanket of double the thickness. Why so?

A. Two blankets are warmer because the air present between the two does not let the heat escape.

Q. 26. Why do we fix a lightning conductor at the top of a building?

A. We do so to protect the building from the destructive effective of lightning. When an electrical  discharge takes place from the clouds above, the lightning conductor provides it an easy conducting path to the earth through the exterior of the building. It protects the building from any damage due to the electrical discharge.

Q. 27. Why do two eyes give better vision than one eye?

A. Our retina is a 2-dimensional surface i.e. cannot give an impression of depth, if only one eye is used. With two eyes, we can see the object from two different points at once, which gives us an idea of solidity or depth, besides that of height and width.

Q. 28. Why does a boatman push the bank with his pole?

A. This can be explained by Newton’s Third Law. The bank will push the boat away from it as action and reaction are always equal and opposite to each other.

Q. 28. Why does a hydrogen balloon rise in air?

A. The weight of a hydrogen balloon is less than that of the air displaced by it. Consequently, the net upward thrust on it carries it upwards.

Q. 29. What is an electrical fuse and why do we use it?

A. An electrical fuse is a metallic wire having a low melting point. When a high current passes through the wire, it melts easily, resulting in breaking of the circuit. The current stops, thus preventing fire, heat and damage to the building.

Q. 30. Why do we lean forward while climbing a hill?

A. We do so to keep the line passing through the centre of gravity vertically downwards, within our feet, which keeps our body in a stable equilibrium.

Q. 31. Why do we see a rainbow only after rains?

A. The water drops suspended in the air act as tiny prisms and when light rays become incident on them, they disperse light into seven colours (VIBGYOR), creating a multi-coloured rainbow.

Q. 32. Why do wet clothes dry more quickly on a warm day than on a cold day?

A. On a warm day, the air contains less moisture than its saturation level and thus it can absorb moisture from wet clothes more rapidly. The high temperature of the air also helps in faster evaporation of water from the wet clothes.

Q. 33. A metal tea-pot always has an ebony handle. Why so?

A. It is because ebony is a bad conductor of heat, so it does not pass heat from the tea pot to our hands.

Q. 34. Why does a diamond shine at night?

A. The light rays entering it get internally reflected from most of the faces due to its high refractive index.  It results in a lot of light coming out from the few reflecting surfaces of the diamond.


Q. 35. Why is our breath visible during the cold season, but not in hot weather?

A. In cold weather, the water vapours present in the exhaled breath condense to become small droplets of water, which become visible to us.

Q. 36. Why is a compass used to indicate direction?

A. The magnetic needle of a compass, under the effect of the earth’s magnetic field, always lies in a North-South direction. By looking at the compass needle, we can find our direction.

Q. 37. Why do we feel very hot in Ladakh in sunshine, but bitterly cold in shade?

A. Some places situated at great heights like Ladakh have a rarefied atmosphere, which provides very little heat from sunshine. Thus the atmosphere remains generally cold, leading to a feeling of extreme cold in the shade.

Q.38. Why does an electric bulb crack with a bang when broken?

A. An electric light bulb has a partial vacuum inside. When it is broken, the air rushes inside to fill the vacuum, thus creating a bang. This bang is called implosion.

Q.39. How is a photograph made, using a camera?

A. A photographic plate is coated with silver salts, which are affected by light. This plate is  exposed to light for a short time, affecting the chemical on it. The plate is developed. (a chemical change which makes the image visible) and then fixed (using hypo solution), which removes the un-reacted chemicals. This negative plate is converted into a positive plate by keeping it in contact with a paper having a similar composition. This is how we get a photo print.

Q. 40. Why does a straight pencil look bent when it is partly immersed in water?

A. When light rays enter another medium, they change their path due to refraction. So the pencil looks bent, when it is partly submerged in water.


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