Does refracting light impart momentum onto the object that is refracting it, for the duration of the refraction?

Submitted by TheFeshy t3_11381g0 in askscience

Let's say we've got light entering a cubic piece of glass, at some angle less than 90 degrees. When it enters, it will be bent at some angle relative to its original path. When it exits, it will resume traveling at its original direction. During this time it will also slow down. (I watched this video explanation as to how this process works.)

Photons have no mass, but they do carry momentum.

During the time the light is inside the glass (let's say it's a very short burst or a very large piece of glass), it's momentum vector is different than when it is traveling in the (for the sake of simplicity) vacuum outside. I have no idea how its momentum is affected by the speed changes, but certainly it would be affected by the direction change. And the conservation of momentum says that this momentum should be exchanged with the glass block.

Now obviously the amounts involved would be minuscule and likely undetectable, given any reasonable source of light and piece of glass. But I'm trying to see if I understand the theory correctly.

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rootofallworlds t1_j8p7aqi wrote

Yes, by conservation of momentum if light is refracted in a way that's not symmetric, momentum must be imparted to the lens. It doesn't even need to be a single pulse - a prism will refract light asymmetrically, and an off-axis section of a spherical-surfaced lens will do it.

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HellKnightRob t1_j9bht20 wrote

Could this be a potential propulsion system in a vacuum? Like I know solar sails are a thing, but could it be plausible to emit a high lumen light directed through a prism for propulsion from the craft itself?

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rootofallworlds t1_j9bipxw wrote

You don’t need a prism. A directional light source produces a recoil force. For a near-perfect beam like a laser you need 300 megawatts per Newton.

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Chemomechanics t1_j8qimev wrote

I did a PhD involving this. Yes, light passing from one medium to another gives the interface a momentum kick. (Edit: example video, not mine.)

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juansinmiedo t1_j8qvb4q wrote

Does it mean that the reflected (or refracted) photon has a longer frequency (less energy)?

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Chemomechanics t1_j8rqj4n wrote

As I recall, the energy is split between absorption, reflection, and refraction. The frequency of the light wave stays the same; its speed changes. Remember that we’re talking about light interacting with matter, rather than lone photons in a vacuum.

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thickener t1_j8rw9kz wrote

This answer got me wondering, if a photon could somehow enter an atmosphere, pass through water, then head back into the vacuum of space, does it “re-accelerate” to “full” lightspeed? How does that work?

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Chemomechanics t1_j8s6uup wrote

No one thing is decelerating or accelerating. The photon–matter interaction propagates slower than c.

See the discussion here and the links within, for example.

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nicemike40 t1_j8rlcng wrote

Fixed link: http://john.maloney.org/Papers/Mechanical%20fluidity%20of%20fully%20suspended%20biological%20cells%20(Maloney%20et%20al%202013).pdf

Or for markdown http://john.maloney.org/Papers/Mechanical%20fluidity%20of%20fully%20suspended%20biological%20cells%20%28Maloney%20et%20al%202013%29.pdf

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Chemomechanics t1_j8rnzx4 wrote

Thank you! I always forget to escape the parentheses on Reddit.

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Local-Program404 t1_j8smu6t wrote

So when the light speeds up when it leaves the medium, where does that come from? The medium? So the net force on the medium is 0 because if gives it back up to the light when it leaves?

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Chemomechanics t1_j8snwzw wrote

The medium plays a part in the momentum transfer, yes.

The net force on a spherical body isn't zero for a single beam because the beam changes shape moving through it, so the refractive details are different on either side.

But for two counterpropagating beams, which is what I used, the left–right forces do balance out, leaving an internal tensile load that stretches a compliant medium. You don't notice this in everyday situations with macroscale objects because they're stiff and the light is weak.

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Local-Program404 t1_j8sp69c wrote

Honestly, this made me think of the emdrive from a few years ago. Interesting phenomenon.

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Fallacy_Spotted t1_j8z6ht8 wrote

Does this mean that the light leaving the medium is of a lower frequency? Can this be setup in a way to increase the frequency and take energy from the medium?

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Chemomechanics t1_j8zh6u2 wrote

The frequency is the same everywhere. (The easiest way to see this may be to note that the electric and magnetic fields must be in sync everywhere, including the interfaces.)

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Fallacy_Spotted t1_j8zhju8 wrote

The light must give some of its energy to the medium so if the frequency doesn't change then where does this energy come from.

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Chemomechanics t1_j8zi3qy wrote

Well, what's different about monochromatic light that's passed through a filter?

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Fallacy_Spotted t1_j8zjks2 wrote

I don't know. I am a normal dude in IT that spends too much time on the internet. I have a middling understanding of this stuff and am curious. If a refractory medium like a lens behaves differently than a filter that would be interesting. 😁

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Chemomechanics t1_j8znwgn wrote

I was just looking for “It’s dimmer.” That’s what provides the energy that’s absorbed in the medium, not any change in frequency.

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GooseMuckle t1_j8qn81o wrote

Yes! This effect is used for laser cooling. The momentum from a photon can be imparted to an atom; if done in the right way a cloud of atoms can be cooled to a few billionths of a degree above absolute zero. One cool application of this technology is optical clocks, which form the most precise clocks in existence.

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