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Homework Help: Measuring energy of photon to within some accuracy

  1. Jul 27, 2009 #1
    1. The problem statement, all variables and given/known data

    Estimate the minimum length of time needed to measure the energy of a photon to an accuracy of 10^(-15) Joules.

    2. Relevant equations

    E = hv, where v is the frequency of the photon and h is Planck's constant in J*s

    Watts = Joules/second

    3. The attempt at a solution

    I can think of two ways to get the units to work out right, but I'm fairly certain they're not right. One would be to ignore the specific frequency of the photon, assuming that E is proportional to Planck's constant in J*s, and dividing to obtain an answer in seconds. But that seems unreasonable to me since the wavelength of the photon should plausibly matter.

    The only other thing that comes to mind straight away is to somehow utilize the definition of watts, to determine how long it would take to get some wattage, but I don't feel that this is right, either.

    What am I missing? I'm sure this is a simple, 30 second problem, I'm just having a mind block... :uhh:
  2. jcsd
  3. Jul 27, 2009 #2
    Are you certain you need to use E = hv?
    I think this is a problem dealing with the Heisenberg uncertainty princple, relating energy with time and planck's constant. Maybe you should read up on it?
  4. Jul 27, 2009 #3
    aha, that's perfect! I can't believe I didn't even think of it. I knew I was missing something easy. It's easy to derive the uncertainty principle with commutators, too, in case one has forgotten that as well. Thanks!

    (See what 4 months of break from school does to the mind?? :grumpy:)
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