Q. Why do we perspire on a hot day?

A. The human body is physiologically conditioned to maintain uniform temperature. When the heat produced in the body becomes excessive and is not dissipated properly, the sweat glands inside the body are stimulated to secrete sweat. Therefore, we perspire on a hot day. However, when sweat evaporates from the body, it produces a cooling effect.

Q. Why does a housewife blacken the bottom of the ‘degchi’ used in the kitchen?

A. The blackened surface absorbs more heat as compared to a polished surface. That is why blackened ‘degchi’ is used.

Q. Why does a coil lying at the bottom of a can filled with water seem to be at lesser distance below the water level than it actually is?
Why does a swimming pool appear less deep than it really is?

A. This is due to the phenomenon of refraction of light. The rays of light coming from the bottom of the pool (or from the coil lying at the bottom of the can) travel from water to air. As the rays pass from denser (water) to rarer (air) medium, they bend away from the normal. When the rays are produced back, they form an image of the coil (bottom of the pool) at a point which is a little above the real position. Therefore, the coil appears to be slightly raised and the pool appears to be less deep than it really is.

Q. One eye is sufficient to form the image of an object. What is the advantage of having two eyes ?

A. The area seen clearly with one eye is comparatively little (approximate range 135°) than with two eyes (range 180°). Two eyes also give better judgement of distances of different objects.
The left eye sees more of the right side of the object and the right eye, more of the left side. Thus, two eyes do not form exactly similar images and the fusion of these two dissimilar images in the brain gives the three dimensional or the stereoscopic vision.

Q. Why do we bring our hands close to the mouth while shouting to somebody at a distance?

A. By bringing the hands close to our mouth, the sound energy is not allowed to spread in all directions, rather being made unidirectional {i.e., directed in a particular direction). Hence, the sound produced is louder.

Q. Why are fuses provided for electric installations?

A. A safety fuse is a wire made up of a material having a low melting point. It is inserted in an electrical circuit as a safety device not to allow excess current to flow through the circuit. When the current exceeds the limiting value, the fuse wire gets heated, melts and breaks the circuit.

Q. A dark blue suit appears black when viewed in candle light. Why?

A. Candle light is deficient in blue colour, whereas yellow colour is in excess. When yellow light falls on the blue suit, blue colour is absorbed by it and, therefore, the appearance of the suit is black.

Q. In summer, white or light- coloured clothes are preferred to dark- coloured clothes. Why?

A. White or light coloured clothes are good reflectors and bad absorbers of heat, whereas dark-coloured clothes are good absorbers of heat. Therefore, in summer, white or light-coloured clothes are preferred, because they absorb very little heat from the Sun’s rays and reflect more. Hence, the person feels more comfortable.

Q. Why is a rainbow seen after rain ?

A. After the rain, some clouds continue to linger in the sky and they contain water droplets. Water droplets act like prisms. The Sun’s rays falling on water droplets suffer dispersion and produce a spectrum. The different colours are viewed in the form of a rainbow.

Q. Why does a green leaf appear green in day light but dark in red light ?

A. A green leaf appears green in daylight because it absorbs all the constituent colours of white light except green which is reflected to us. A green leaf appears dark when viewed in red light, because it has the property of absorbing all colours except green. As red rays falling on the leaf are absorbed by it, the leaf appears dark.

Q. Although each eye perceives a separate image, we do not see everything double. Why ?

A. The axes of the two eyes are directed towards the same object. Therefore, there appears to be only one object. The two separate images formed b)’ two eyes get fused in the brain. The optic nerves lead to the same point in the brain producing only one sensation. Hence, we see only one object with two eyes. It may be pointed out that we also hear only one sound with two ears.

Q. Why is it difficult to thread a needle with only one eye ?

A. It is difficult to estimate the relative distance between the thread and the walls of the hole of the needle with one eye. The thread, therefore, passes not through the needle but in front or behind the hole of needle.
With two eyes, however, it is easy to judge the relative distance between two points. Hence, it becomes easy to thread the needle with two eyes.

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