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Photon energy question

  1. Mar 9, 2009 #1
    1. The problem statement, all variables and given/known data

    A certain laser outputs pure blue light (photon energy 3.1 eV) with power 800 milliwatts (0.8 watts). How many photons per second does this laser emit?

    2. Relevant equations

    1 Ws = 6.24151e18
    might need E = hv or E = (hc)/fequency where h is 6.6e-34

    3. The attempt at a solution

    not sure if i did it write but i took the .8 multiplied by the 6.24e18 (this is converting Ws to eV) then i x this number by the 3.1 eV to get 1.5479e19
    this should be eV/s. then i am lost. not sure if that is even right so any help would be great!!
  2. jcsd
  3. Mar 10, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    To solve this type of problem, it helps to think about units carefully (and properly!).

    Actually, it says 0.8 is in units of W, or equivalently J/s. If we include the units in the calculation, we can see what are the units of the result:
    (0.8 J/s) x (6.24e18 eV/J) = _____?​

    Here, it helps to think of it as 3.1 eV/photon (i.e, eV per photon, the energy that each photon has).

    Like I did before, include the units in the multiplication and see what the result is.

    I hope my suggestions help you out. If you follow them, compare the units in the answer with what is being asked (highlighted in blue, near the beginning of my post.)
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