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

AI Thread Summary
The discussion centers on the change in momentum of light when it strikes a reflective surface at an angle, with conflicting equations presented for calculating this change. Participants debate the correct formula, with one asserting P=2IcosA/C while another suggests P=2Icos^2(A)/C. The conversation highlights the importance of referencing sources when quoting results and addresses the concept of mass in relation to photons, emphasizing that photons are massless and should use energy-momentum relations instead. Additionally, the impact of Doppler shift on momentum calculations is noted when surfaces approach the speed of light. The need for clarity in physics discussions and the correct application of relativistic principles is underscored.
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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