Question by thisislizz: Radiation pressure, photon momentum, and solar sails?
So I understand that a photon can have momentum, but how is this momentum is transferred to an object off of which it reflects?
Once the photon bounces off an object, it does not slow down or change wavelength or frequency, and therefore does not lose momentum or energy, so how does it transfer any of its energy to, say, a solar sail? Doesn’t this violate conservation of energy?
The momentum changes because direction changes, but velocity change does not equate to momentum loss. Direction change would require an outside force, and therefore the photon has not lost energy, and has not transferred any energy to the object off of which it bounced.
Since posting this, I was reading on the nature of reflection, and it seems to clear this up a bit. Feynman talks about radiation pressure in “Lectures on Physics.” He explains that as light is an electromagnetic wave (a time-varying electric and magnetic field), these fields interact with the electrons in a material, and the electrons begin to move around. Moving electrons give off electromagnetic waves of their own, in all directions. These scattered EM waves are what’s reflected to your eyes.
Radiation pressure, as I understand it, comes not from the momentum or energy of photons, but from electric charge! So now we’re in a whole new ballpark.
I’m still not clear on what happens to the original EM after it interacts with the object.
Furthermore, this question still isn’t answered until the source of charges and magnetism can be explained.
Ok I misinterpreted the interaction of light and object.
When light is shining on a charge, the electric field component of light drives the charge up and down. Now that the charge is moving, the magnetic field now has a force on the charge in the direction of the light beam. That is radiation pressure, F=qvB.
Light can impart momentum to an object, but only if it is absorbed. In our case of reflection, there is no momentum exchange.
I still feel confused though.
Incidentally, in my readings I have found tons of articles claiming that the photon transfers momentum to the object because the light changes direction (from c to -c, which is why it is said that reflected light imparts double the pressure that absorbed light does because it has changed twice its momentum), but there is no explanation of what kind of force or energy is required to cause this direction change or an explanation of the source of said force, and furthermore, what observable effect this momentum change has on the photon. Since the energy of a photon is E=hf (Planck’s constant X frequency), the photon has not lost energy, so what energy did the photon transfer to the object if you were to maintain this theory?
Best answer:
Answer by Shawn F. Jacobson
When the photon bounces/reflects off an object its momentum does change because its velocity does change. Thinking in classical terms, the speed of light is constant, not the direction (which changes the velocity). If it reflects completely its velocity has gone from c to -c. If it is absorbed, its velocity goes from c to 0,
Either way the momentum changes and that momentum is imparted on the object it comes in contact. I remember common questions from physics books that would want specifications for a laser that could suspend a block of a certain weight solely because of the radiation pressure of the light. I always thought those were fun questions.
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Good Points. I’ve been looking around for a little while and will continue to do so later. I found this sight that talks quite a bit about light but seems to disagree with light being able to exert a pressure at all (or be bent by gravity and such…) http://www.hyperflight.com/ It might be worthwhile to checkout though.
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