Answer the following:
(a) Name the em waves which are used for the treatment of certain forms of cancer. Write their frequency range.
(b) Thin ozone layer on top of stratosphere is crucial for human survival. Why?
(c) An em wave exerts pressure on the surface on which it is incident. Justify.

Give reasons for the following: (i) Long distance radio broadcasts use short-wave bands. (ii) The small ozone layer on top of the stratosphere is crucial for human survival. (iii) Satellites are used for long distance TV transmission.

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? 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?

Rakesh and Rajesh are 8th class students. They are fond of watching cricket match, particularly when it is played between Australia and India. They observed that most of the players, when they are in the field, apply a cream on their face. They did not know its reason. One day they asked this question to their teacher. The teacher thought it to be a good question and explained the reason for applying this cream to the whole class. Based on this paragraph, answer the following questions: (i) In your opinion, what explanation did the teacher offer to the students in the class? (ii) Why is small ozone layer on top of the stratosphere considered crucial for human survival? (iii) Write any two values displayed by Rakesh and Rajesh and their class teacher?