Light Reflection Momentum - P=2Icos^2(A)/C

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Discussion Overview

The discussion revolves around the change in momentum of light when it strikes a reflective surface at an angle, specifically examining the formulas P=2Icos^2(A)/C and P=2Icos(A)/C. Participants explore the implications of these equations in the context of light reflection, momentum, and the behavior of photons.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the change in momentum for light reflecting off a surface should be represented by P=2Icos(A)/C, while others propose P=2Icos^2(A)/C.
  • One participant mentions the need to consider the Doppler shift if the reflective surface is moving at relativistic speeds, suggesting that this could modify the results.
  • There is a discussion about the definition of mass in the context of photons, with some participants indicating that mass cannot be associated with photons and suggesting the use of mass-energy equivalence instead.
  • Participants express uncertainty about the sources of their claims, with one participant offering to provide an image of the derivation of their proposed formula.
  • The concept of relativistic mass is brought up, with a participant cautioning against its use and discussing the relationship between energy, momentum, and mass.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct formula for the change in momentum of light upon reflection, and multiple competing views remain regarding the definitions and implications of mass in this context.

Contextual Notes

Participants note the importance of providing sources for claims made, and there is an acknowledgment of the complexities involved in discussing the behavior of light and momentum, particularly in relativistic scenarios.

Neeraj
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Even if a beam of light strikes a reflective surface at an angle 'A', the change in momentum should be 2mc, P=2IcosA/C but I find it P= 2Icos^2(A)/C.
 
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Neeraj said:
Even if a beam of light strikes a reflective surface at an angle 'A', the change in momentum should be 2mc, P=2IcosA/C but I find it P= 2Icos^2(A)/C.
Where did you see this?...I believe you are correct
 
hutchphd said:
Where did you see this?...I believe you are correct
Which one? I believe speed of light is constant in every frame we presume. I found it in a book.
 
If the velocity of the surface is large (near c) there will also be an appreciable effect from the Doppler shift, so the result will need modification. If you quote a result it is good form to provide the source (I.e. which book..there are quite a few!).

Also you say p=mc ...what is m? This is not correct.
 
Last edited:
hutchphd said:
If the velocity of the surface is large (near c) there will also be an appreciable effect from the Doppler shift, so the result will need modification. If you quote a result it is good form to provide the source (I.e. which book..there are quite a few!).

Also you say p=mc ...what is m? This is not correct.
Here we require the change in momentum to find the force and then the pressure, and since we can't associate mass to photons, mass energy equivalence can be used to replace mass (p.s. It was explained in photoelectric effect to find out radiation pressure)I don't remember the exact source, I just wanted to confirm if I am right or not.
 
Neeraj said:
Here we require the change in momentum to find the force and then the pressure, and since we can't associate mass to photons, mass energy equivalence can be used to replace mass (p.s. It was explained in photoelectric effect to find out radiation pressure)I don't remember the exact source, I just wanted to confirm if I am right or not.
If you can check, I can send an image on how they derived it.
 
Neeraj said:
If you can check, I can send an image on how they derived it.
If you are using relativistic mass (*shudder*) then mc^2 = E = pc. Divide by c to get mc = E/c = p
 
jbriggs444 said:
If you are using relativistic mass (*shudder*) then mc^2 = E = pc. Divide by c to get mc = E/c = p
Photons are massless so in order to avoid the term m we can use debroglie's dual nature theory or whatever you just said
 

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