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Need some help with half life

  1. Dec 15, 2015 #1
    So I'm getting ready to buy some Tritium gun sights that will glow in the dark. The advertisement says Tritium has a half life of 12.3 so the sights will be about half as bright (50%) after 12.3 years.

    I'm wondering how bright they will be after 6 years and 2 months...will they be one quarter as bright?

  2. jcsd
  3. Dec 15, 2015 #2
    Simple estimate would be 75 %, but 70.7 % (square root of one half) is accurate.
  4. Dec 15, 2015 #3


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    That's 1/4 LESS bright, not "as" bright.
  5. Dec 18, 2015 #4


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    Yes - "Threequarters as bright".
    This way of stating things is getting very common - particularly when describing the cost of things but it is potentially very confusing. It is a shame that decreases are not expressed in fractions of the original (or possibly percentage).
    "Reduced to 75%" or "reduced to 3/4" of the original can't be misconstrued.
    Even worse is "A hundred times less", instead of "one hundredth". This is used throughout the media and must be a nightmare for children, learning to do arithmetic. It's also a great way to hoodwink innocent consumers and voters.
  6. Dec 18, 2015 #5
    Don't get me started. My local weatherman told us this morning "It's 46 degrees now, that's twice the normal 23 degree morning temperature for December 18..."
  7. Dec 18, 2015 #6


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    Could be worse. It could be half the normal temperature for December 18 ...
  8. Dec 18, 2015 #7


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    And answer me this one: then the bank manager tells you that the interest he's charging you has only gone up by 1% (i.e. from 1% to 2%) how has he the nerve not to admit that the cost has actually increased by 100%?
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