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WHY HALF LIFE?



  HALF LIFE:  We have studied 'Radioactivity' in our school days.  Some unstable atoms emit radiations and become the atoms of another stable element.  For example, 'Uranium' is radioactive and step by step becomes 'lead'.

    This we call 'exponential decay'.  When the overhead tank' is full, water will come out through the tap forcibly with high presssure.  As the water level decreases in the tank, the presssure goes down and finally water drips through the pipe.  In a 'exponential decay', the process will take infinite time to complete.  Hence, to get the idea of the 'age' or life time of radioactivve element, we measure half life'. In the case of 'Radioactivity' the 'element of chance' is also present.

    What is half life?  In the case of radioactivity, the time taken for the half of the original atoms to become atoms of another lelment.  'OR'.  The time taken for the 'effects of anything' to become half.  For example:  When your fever will reduce by half?

     The half life of Radioactive carbon-14 is nearly 6000 years.  The C-14 decays into carbon -12.  All living beings take in carbon-14 while living,  through respiration.  When the life ceases, taking C-14 also stops.  Then the disintegration starts.

     Suppose an archeologist finds an old bone.  Using an instrument called "Geiger Muller counter", he finds that there are only 6 carbon-14 atoms that emit radiations (just an assumption).  He knows that a fresh bone of same nature normally has 100 C-14 atoms (say).

      Time                         No. of C-14 atoms
Initial: zero                          100
half life   6000years              50
                12000 years          25
                 18000 years         12.5
                 24000 years           6.25

     Hence the animal or the man that carried this particular bone lived nearly 24000 years ago.  This is how, we estimate the age of pre-historic objects.  It is called 'Radio carbon dating'.

     Suppose a drug is injected into our body, its effect has half-life.  Our skill, knowledge also have half-life.

     Scientists calculated 'age of the earth' in this way.  Radioactive uranium turns into 'lead' atoms step by step.  Its half life is million to trillions of years.  People located oldest rock and counted radioactive uranium atoms.  They estimated the age of earth as 4.543 billion years.  If we assume or  compress 'age of earth' as 24 hours, we human beings present on the earth only for the last 4 seconds.

      Thus, age of fossils, relics, historic artefacts can be measured using "half-life".     
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