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MCQ and Study notes Air pollution is described as any change in the composition of air either by physical or chemical methods so as to cause harmful effects on health. The substances which contaminate the air are called air pollutants. Sources of Air Pollution are : 1. Natural sources 2. Man-made (anthropogenic) sources. Natural sources: Volcanic eruption, forest fire, sea salt sprays, biological decay, photochemical oxidation of terpenes oil , marshes, pollengrains, spores, etc., Man made sources : Industrial emissions, vehicles, aeroplanes, power stations and burning of fuels, etc., Mainly air pollution is caused due to burning of fuels to run vehicles, and from chimneys of factories and power stations. The Green House Effect : Some of the infrared radiation from the earth passes through the atmosphere but most is absorbed and re-emitted in all directions by greenhouse gas molecules and clouds. This warms up the Earth’s surface and the lower atmosphe

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CBSE PHYSICS: IX Physics Sound NCERT Solutions Explain how sound is produced by your school bell. When the bell continues to move forward and backward, it creates a series of compressions and rarefactions. In this  way a bell  produce the sound . Why sound waves are called mechanical waves? Sound waves need the material medium to vibrate and propagate. Hence, these waves are known as mechanical waves. Sound waves propagate through a medium because of the interaction of the particles present in that medium. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend? Sound needs a medium to propagate. Since there is no material medium on the moon due to absence of atmosphere, you cannot hear any sound on the moon. Page 166 : Which wave property determines (a) loudness, (b) pitch? (a) Amplitude of sound wave determines the loudness of a sound. The amplitude of a sound directly proportional to amplitude of wave. If the amplitude of a sou