8.15 Answer the following questions:

(a) Long distance radio broadcasts use short-wave bands. Why?

(b) It is necessary to use satellites for long distance TV transmission. Why?

(c) Optical and radiotelescopes are built on the ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?

(d) The small ozone layer on top of the stratosphere is crucial for human survival. Why?

(e) If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now?

(f) Some scientists have predicted that a global nuclear war on the earth would be followed by a severe ‘nuclear winter’ with a devastating effect on life on earth. What might be the basis of this prediction?

<p><strong>8.15 </strong>Long distance radio broadcasts use shortwave bands because only these bands can be refracted by ionosphere.</p><p>It is necessary to use satellites for long distance TV transmissions because television signals are of high frequencies and high energies. Thus these signals are not reflected by ionosphere. Hence, satellites are helpful in reflecting TV signals. Also they help in long distance TV transmissions.</p><p>With reference to X-ray astronomy, X-rays are absorbed by the atmosphere. However, visible and radio waves can penetrate. Hence, optical and radio telescopes are built on the ground, while X-ray astronomy is possible only with the help of satellites orbiting the Earth.</p><p>The small ozone layer on the top of the atmosphere is crucial for human survival because it absorbs harmful ultraviolet radiation present in sunlight and prevents from reaching Earth's surface.</p><p>In the absence of an atmosphere, there would be no greenhouse effect on the surface of the Earth. As a result, the temperature of the Earth decreases rapidly, making it chilly and difficult for human survival.</p><p>A global nuclear war on the surface of the Earth would have disastrous consequences. Post nuclear war, the Earth will experience severe winter as the war will produce clouds of smoke that would cover maximum parts of the sky, thereby preventing solar light to reach the atmosphere. Also, it will lead to the depletion of the ozone layer.</p>

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