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Force exerted by a laser

  1. Apr 19, 2017 #1
    1. The problem statement, all variables and given/known data
    A target is hit by a laser pulse. The frequency of the pulse is in the visible spectrum, it has duration ##5\times 10^{−15}## s and it contains approximately ##10^{19}## photons. Estimate the force exerted by the pulse on the target, given the pulse is completely absorbed by the target.

    2. Relevant equations


    3. The attempt at a solution
    I think a reasonable wavelength in the visible spectrum would be ##550##nm. I can calculate the total energy of the photons that hit the target by using ##E = \frac{hc}{\lambda}## and then multiplying by ##10^{19}##. I get ##3.6\times 10^{-19}##J, and don't know where to go from there.

    Alternatively I suppose I could use force = change in momentum / change in time. Then I'd use ##p = \hbar k## where ##k = \frac{2\pi}{\lambda}##. If that's the correct approach, to get the total change in momentum I just multiply the momentum expression by ##10^{19}##, right?

    Thanks for any help!
     
  2. jcsd
  3. Apr 19, 2017 #2

    DrClaude

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    Staff: Mentor

    That will give you the total momentum. You'll then have to convert it to a force.
     
  4. Apr 19, 2017 #3
    By dividing the total momentum by the duration of the pulse?
     
  5. Apr 19, 2017 #4

    haruspex

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    Yes.
     
  6. Apr 20, 2017 #5
    Brilliant, thanks! :)
     
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