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Pressure of Light

  1. Oct 16, 2012 #1
    Hi PF,

    I have a question about light pressure: Let's take a laser beam. Each photon in the laser beam has a momentum p=h/λ.

    If photons have pressure, dp/dt must be non-zero but, it is zero.

    Where do I make mistake?

    Thank you for answers.
  2. jcsd
  3. Oct 16, 2012 #2
    What makes you think dp/dt is zero?

    Do you just mean that the photon has a constant momentum as the beam travels through space? If so, then the answer is: the change in momentum does not happen when the photon is just hurtling through space, it happens when the photon hits a piece of matter and interacts with it. Even a plain old billiard ball rolling across a table has a constant momentum--it's when it hits the other balls that its momentum changes.
  4. Oct 16, 2012 #3
    Oh, okey. Thank you for your answer :) I understand.
  5. Oct 17, 2012 #4
    pressure of light is owed to it's magnetic field.when light will fall on a charged particle ,the electric field of light will give a motion in [tex]\vec{E}[/tex] direction and then the magnetic field will act .it is easily shown by the curl of (v×B) that the force is in direction of propagation.(it does not depend on the sign of charge)
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