SCIENTIFIC REASONING QUESTIONS FOR ALL COMPETITIVE EXAMS

71) Why is mirage formes in the deserts?

Ans: A mirage is formed owing to total internal reflection. To the observer at a distance, the reflected image of the object appears behind the reflecting surface, as if the object were in front of it, but

actually it is just an illusion.

72) What will be the colour of grass in blue light?

Ans: Grass will appear blackish in colour in blue light because it has the property of absorbing all other colours except its own colour. The blue rays falling on grass will be absorbed by it, and it will appear dark coloured.

73) A bird perches on a bare high power line and nothing happens to it. A man standing on the

ground touches the same line and gets a fatal shock. Why?

Ans: When bird perches on a live high power line, no current passes through the body of bird because there is no potential difference between live wire and the body of bird as the potential of bird‘s body is the same as that of wire.When a man standing on the ground touches the same wire then due to a large potential difference between his hands and feet, a large current flows through his body to the earth.

74) During lightning it is safest to be inside a car rather than under a tree or in the open. Give reason.

Ans: When a person is in the open or under the tree, the lightning passes through his body to the

earth. On the other hand, for a person inside a car, the car provider shielding and the electric field

inside the car is zero, thus lightning does not affect the person inside it. The lightning actually passes through the metallic body of the car to the earth without affecting the person sitting inside it.

75) Why is earthing desirable for electric appliances?

Ans: Earthing helps the current move into the earth in the event of short-circuit, without giving a

shock to the user.

76) Why does a perspiring man feel relief when air flows by his side?

Ans: The air flowing by the side of a perspiring man quickens the pace of evaporation of perspiration from the body of that person, and the resultant loss of heat from his body causes the cooling sensation which provides relief to him.

77) Why is magnet always made of soft iron?

Ans: Magnet is always made of soft iron because ‘magnetisation’and ‘demagnetisation’ both are

possible in soft iron.

78) Why are telephone wires between two poles kept loose?

Ans: Telephone wires are kept loose between two poles so that they are protected from being broken due to expansion and contraction caused by the change of temperature in summers and winters.

79) X-rays penetrate through the flesh but not through bones. Why?

Ans: The penetrating power of X-rays depends upon the potential difference between the cathode and the anode of X-ray tube. The X-rays produced can penetrate through light element like flesh of human body but they are unable to penetrate through heavier elements like bones.

80) What is a black hole? Why is it called so?

Ans: A black hole is a super dense planetary material formed due to the death of a star of mass more than five solar masses. It is called black hole because any particle or photon approaching its surface is just swallowed by it.It appears black, as radiation is neither emitted nor reflected by it.