Solving the Mystery of Light Pressure | Physics Forum

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    Light Pressure
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Discussion Overview

The discussion revolves around the concept of light pressure, specifically in the context of a laser beam and the momentum of photons. Participants explore the implications of photon momentum and its interaction with matter, as well as the underlying mechanisms that contribute to light pressure.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the assumption that the rate of change of momentum (dp/dt) is zero for photons in a laser beam, suggesting that momentum changes occur only upon interaction with matter.
  • Another participant agrees with the clarification about momentum being constant while photons travel through space, emphasizing that momentum change is associated with interactions.
  • A different perspective is introduced, stating that the pressure of light is related to its magnetic field and the interaction of light with charged particles, indicating a complex interplay of electric and magnetic fields in generating light pressure.

Areas of Agreement / Disagreement

Participants express differing views on the nature of momentum change for photons, with some agreeing on the interaction aspect while others introduce additional factors related to electromagnetic fields. The discussion remains unresolved regarding the complete understanding of light pressure mechanisms.

Contextual Notes

There are limitations in the assumptions made about momentum change and the role of electromagnetic fields, which are not fully explored or defined in the discussion.

coki2000
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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.
 
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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.
 
Chopin said:
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.

Oh, okey. Thank you for your answer :) I understand.
 
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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