________________
compounds and dust particles. The composition of earth's atmoshphere varies very slightly with altitude. It remains same up to a height of 100 km but its density goes on decreasing as we go up. The earth's atmosphere has no sharp boundary. It has been divided into a number
of regions (or layers) as shown in Fig. They are as explained blow : 1. Troposphere. This region extends up to 12 km from the surface of
the earth. Its density varies from 1 kg mo at the surface of earth to 0.1 kg m-3 at the top of this layer. All the water vapours of the atmosphere are contained in this layer. In this part of atmosphere, temperature decreases with height from 290 K to 220 K. Stratosphere. This region of atmosphere extends from 12 km from the surface of earth upto 50 km and it forms practically clear sky. The density of atmosphere in this region varies from 0.1 kg m-3 to 10-3 kg m-3 and the temperature varies from 220 K to 280 K. At the upper extreme of this region, there is a layer of ozone between 30 to 50 km from the surface of earth, in which most of the atmospheric ozone is concentrated. It is responsible for absorbing a large proportion of ultra-violet radiation radiated by the sun and this layer protects the life on earth from its harmful effects. The ozone layer is also called
ozonosphere. 3. Mesosphere. The resion of earth's atmosphere between 50 to 80 km
from the surface fo earth is called mesosphere. The density of the atmosphere in this region varies from 10-3 kg m-3 to 105 kg m- and the temperature varies from 290 K to 180 K." Text-book of Physics, (Std XII), Part II, Pages 100, 101"Electromagnetic Radiations and Earth's Atmosphere Electromagnetic radiations passing through different media suffer effects like scattering, refraction, reflection, polarization and absorption. The effects produced by the electromagnetic radiations of different wavelengths during their passage through the earth's atmosphere are of particular importance to us. The following points are to be noted : (1) As you go above earth's surface, density of the atmosphere goes
on decreasing, but there is no sharp limit to the atmosphere
above. (2) Upper layers of the atmosphere are partially ionized; .i.e. they
have atoms ionized to electrons and positive ions. This layer
is called the ionosphere. (3) Layers of the atmosphere below the ionosphere are mostly of
neutral atoms and molecules.
(b)
137
Jain Educationa International
For Personal and Private Use Only
www.jainelibrary.org
