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Nice easy question about Photons.

  1. Jun 15, 2011 #1
    Estimate how many photons of visible light (500 nm) are emitted by a 100 W Bulb.

    I get 2.5[itex]\times[/itex]10[itex]^{20}[/itex] Photons/Second

    Am I correct?

    Thank you.

    EDIT: Added units.
    Last edited: Jun 15, 2011
  2. jcsd
  3. Jun 15, 2011 #2


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    Two questions:

    (1) Do you mean 2.5*10^20 photons/second?

    (2) Does a 100W bulb emit 100W of light energy, or does it use 100W of electrical energy (or both)?
  4. Jun 15, 2011 #3
    (1) yes, sorry. (2) I assume it means 100w of light energy. But what would the answer be if it was electrical?
  5. Jun 15, 2011 #4


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    It doesn't mean 100W of light energy. An incandescent bulb is only about 4% efficient, and so only puts out about 4W of light energy.
  6. Jun 15, 2011 #5
    Ok well these are practice questions written by a physics professor. The answer he has written at the bottom of the questions is approximately 10^20 photons/second. I just wanted to know where i had gone wrong if I had. The question is as above and its not something you need to pick apart I was just asking for a little bit of help is all. It does mean 100W of light energy. So am I correct or am I incorrect? The idea is for us to learn and understand the formula.

    Substituting in the value of 4W give you 10^18 photons/second which is much further out.
  7. Jun 15, 2011 #6

    Vanadium 50

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    Then please post this in the appropriate section.
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