How does a photon reflect off a smooth surface?

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    Photon Reflection
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Discussion Overview

The discussion revolves around the behavior of photons when they encounter a smooth surface, specifically focusing on the mechanics of reflection and the implications of a photon's masslessness on its acceleration and momentum. The conversation touches on concepts from optics and classical mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how a massless photon can reflect off a surface without being accelerated, suggesting that it should pass through instead.
  • Another participant clarifies that while a photon's speed cannot change, its direction can, which alters its momentum.
  • A further contribution emphasizes that a photon's energy and momentum can change, while its speed remains constant, and relates this to Newton's third law regarding the interaction with the mirror.
  • One participant challenges the previous explanations by asserting that changing the direction of a photon's velocity constitutes acceleration, implying that the reflected photon may not be the same as the incident photon.
  • Another participant supports this view, suggesting that the photon may be destroyed and a new one created upon reflection, referencing a prior discussion for further context.

Areas of Agreement / Disagreement

Participants express differing views on the nature of photons during reflection, particularly regarding whether the same photon is retained or altered in the process. There is no consensus on the implications of masslessness and acceleration in this context.

Contextual Notes

Some discussions hinge on the interpretation of classical mechanics versus quantum mechanics, particularly in how photons are conceptualized in terms of individuality and behavior upon reflection.

aayushgsa
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Hello,
I was just reading optics and I thought that if photon doesn't has mass it can't be accelerated..
If it is so then how it gets reflected off a smooth surface, why don't it just passes from the plane. As any reaction force by the plane couldn't accelerate it , it mustn't rebond as rebounding means accelerating.
 
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You cannot change a photon's speed, but you can change its direction of motion, and thereby change its momentum.
 
Exactly. You can change the photon's energy and momentum, but the speed is the same, since the speed of light is a fundamental constant. What changes is the frequency of light (for energy) or the wavelength of the light (for momentum).

Newton's third law does apply here, and since the momentum of the photon is changed, so is the momentum of the mirror (in the opposite direction). If you had enough light, you could push the mirror with a noticeable force. This is what's behind how solar sails work.

Of course, things get iffy when you start treating photons like massless billiard balls moving at the speed of light, but unless you are really looking at an individual photon and not beams of trillions of them in barely visible laser beams, and the mirror has no features small enough to be even close to the wavelength of light, a classical perspective works often enough.

Hope this helps:)
 
jtbell said:
You cannot change a photon's speed, but you can change its direction of motion, and thereby change its momentum.
Sorry jtbell but I don't think this is an answer for the OP. You have stated that a photon's speed can't vary, but you didn't say anything about its velocity and since velocity is a vector, simply changing the velocity's direction there is an acceleration.
Unless the photon reflected actually it's not the same photon sent to the surface...
 
Yes, that's my point @lightarrow
 
It's probably meaningless to attribute individuality to single photons, they are not as little balls; even if the mirror doesn't absorb the photon in the sense that its energy doesn't vary at all, I think the photon is actually destroyed and a new one re-created by the mirror, at least this is what vanesch suggested in this thread:

https://www.physicsforums.com/showthread.php?t=151977
 
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