Question

The ozone layer is important because it:
(A) Is used by living organisms for respiration.
(B) Increases the temperature of the earth.
(C) Reduces the amount of ultraviolet rays reaching the earth.
(D) Reflects the heat from the earth back into the atmosphere.

Answer 1

Hint: (i) Ozone or trioxygen is a stable allotrope of oxygen but is much less stable than the diatomic allotrope \[{{\text{O}}_2}\] . It is an inorganic molecule and has the chemical formula \[{{\text{O}}_{\text{3}}}\] . Ozone is formed from dioxygen by a photochemical reaction which decomposes the dioxygen molecule into reactive atomic oxygen.
$
  {{\text{O}}_{\text{2}}}{\text{ + hv}}\overset {} \leftrightarrows {\text{O + O}} \\
    \\
 $
(ii) This atomic oxygen then combines with molecules of dioxygen to form ozone. The third body M then absorbs the excess energy liberated and thereby stabilizes the ozone molecule.${{\text{O}}_{\text{2}}}{\text{ + O + M}}\left( {{{\text{N}}_{\text{2}}}{\text{or}}{{\text{O}}_{\text{2}}}} \right) \to {{\text{O}}_{\text{3}}}{\text{ + M}}$
(iii) It is present in very low concentrations throughout the earth’s atmosphere.

Complete step by step answer:
The stratosphere layer at the height of 15 km is rich with ozone. Ozone forms a very well marked ozone layer called ozonosphere within the stratosphere layer. Thus, the concentration of ozone is highest in the stratosphere.
Ozone accounts for only three parts in ten millions of earth’s atmosphere but plays a very important role in the radiation balance of the earth. Ozone acts as a protective shield for life on the earth from the injurious effects of the sun’s ultraviolet ray. It absorbs the harmful ultraviolet radiation from the sun and raises the temperature.
If this radiation is not absorbed, then it will cause sunburn in humans and also lead to DNA damage in plant and animal tissues. Thus, the ozone layer is very important to life on earth as it extends the absorption of ultraviolet rays by ordinary oxygen.

Therefore, option C is correct.

Note: The depletion of this ozone layer by chlorofluorocarbons or by decomposition of oxides of nitrogen leads to the increase in the entry of uv rays to the earth’s biosphere which is dangerous to human life. Ultraviolet radiation causes leukemia, breast cancer, skin cancer and when absorbed by the cornea of the eye, it causes cataracts. It also leads to the death of micro-phytoplanktons.