- #1
Fugie Runner
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Do photons reflected off prisms exert directional force however small as they are reflected off the hypotenuse side of a right triangle shaped prism?
Reflection of Photons on Prisms: Directional Force?
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- Thread starter Fugie Runner
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Discussion Overview
The discussion centers on whether photons reflected off prisms exert a directional force, particularly in the context of a right triangle-shaped prism. It explores concepts related to momentum, reflection, and the behavior of photons in quantum mechanics and quantum field theory.
Discussion Character
- Exploratory, Technical explanation, Debate/contested
Main Points Raised
- Some participants propose that photons, when reflected off a prism, exert a directional force due to the conservation of momentum.
- One participant emphasizes that light carries momentum as a vector quantity, suggesting that a change in direction necessitates a recoil to conserve total momentum.
- Another participant questions the notion of photons bouncing off surfaces, noting that photons are massless and cannot be put at rest, implying they must be absorbed and re-emitted instead.
- A later reply accepts the idea that absorption and re-emission can be mathematically equivalent to reflection but seeks clarification on how this is treated in quantum mechanics or quantum field theory.
- There is a query regarding whether the momentum from the reflection is absorbed by a single atom or the entire prism, drawing a parallel to the Mossbauer effect.
Areas of Agreement / Disagreement
Participants express differing views on the nature of photon reflection and its implications for momentum conservation, indicating that multiple competing perspectives remain unresolved.
Contextual Notes
The discussion includes assumptions about the behavior of photons and their interaction with materials, which may depend on specific definitions and interpretations in quantum mechanics.
- #2
jtbell
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Light carries momentum, which is a vector quantity, that is, it has both magnitude and direction. Therefore, if a beam of light changes direction, something must recoil in order for the total momentum to be conserved. This is true in both the classical and quantum models of light.
- #3
humanino
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I fully agree with jtbell answers. I would like to point out that the original question suggests that photons can somehow bounce off a surface. Technically however, photons being massless cannot be put at rest, so must be absorbed and re-emitted. Maybe just nitpicking 
- #4
jimgraber
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humanino said:I fully agree with jtbell answers. I would like to point out that the original question suggests that photons can somehow bounce off a surface. Technically however, photons being massless cannot be put at rest, so must be absorbed and re-emitted. Maybe just nitpicking
Wow, I never thought a reflection or an elastic scatter involved a state of rest. However, I can accept that absorption and re-emission is mathematically equivalent to a reflection or a scatter. Is this really the way it is treated in QM or QFT? Also, is the momentum of the reflection absorbed by a single atom, or by the entire prism or mirror, similar to the Mossbauer effect?
Inquiring minds want to know. TIA.
Jim Graber
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