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common_sensei t1_j8hpj47 wrote

Reply to comment by leftoutoctopus in Light traveling through a medium that slows it. Does the same photon emerge? by TheGandPTurtle

I like this Fermilab video for explaining light slowing in a medium: https://youtu.be/CUjt36SD3h8

You can think of it as a wave moving through water with a bunch of ping pong balls. As the wave lifts and drops the ping pong balls they resist the acceleration, and that makes a little inverse ripple within the bigger wave. The big wave and the little ripples stack together into a slower wave, but the energy doesn't change, so once the wave moves past the ping pong balls it goes back to the same speed and height.

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leftoutoctopus t1_j8hvp8a wrote

Incredible video, but I still don't understand how does a wave not loses energy as it passes through mediums, or the energy it loses is negligible since it has so little? What happens? I've understood something and now I understand even less.

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KillerCodeMonky t1_j8hzclh wrote

A wave doesn't contain energy, it is energy. And it would only lose that energy by doing work, same as anything else. So what work do you think it is doing when it passes through a medium?

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Stillcant t1_j8i24m2 wrote

Well the person above said the wave is resisted by the medium, and the wave moves the medium (the ping pong balls) as it passes. That sounds like work in the analogy used at least.

And could a photon not shift to a lower energy wavelength? Different photons have different energy I thought?

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KillerCodeMonky t1_j8i455z wrote

Here on earth, a ball resists being lifted due to gravity. It also returns all that energy when it's dropped again.

A wave that lifts a bunch of ping pong balls on it's leading edge, then drops them on it's trailing edge where the energy is returned to the wave, has done net zero work.

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grahampositive t1_j8ici68 wrote

This is where the analogy breaks down though. A water wave lifting a ping pong ball and returning it to it's initial position has lost energy. The ping pong ball pushes against air as it moves upwards, and the resulting ball-air collisions generate heat which is lost to entropy.

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KrombopulosThe2nd t1_j8i7auz wrote

Well here on earth, is it possible to lift anything with 100% efficiency? Shouldn't there be some loss of energy

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KillerCodeMonky t1_j8ia5jn wrote

Yes of course these are not perfect processes. Otherwise an undersea earthquake would create a tsunami on all of its coastlines. Energy is lost or made non-coherent in a variety of imperfections, including heat and scattering. In the case of ocean waves, they are typically created and recharged by the wind as they move along.

I mostly wanted to make the point that a wave is not displacing its medium, but simply moving through it. A wave is nothing but water and energy. There's nothing there displacing the water to do work. The water moves around due to the energy, but in a way which is generally neutral in terms of work actually done. A wave hitting the shoreline really just transfers from moving through the water to moving through the land.

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DamionFury t1_j8ijw6m wrote

Work can turn out to be one of the less intuitive aspects of physics. For example, magnetic fields cannot do any work because they act orthogonally to the direction of motion, yet it certainly looks like work when you use an electromagnet to lift an object and make it float. I wish I could remember the explanation my Electromagnetism professor gave me for what is actually doing the work in that scenario.

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Parrek t1_j8j1x62 wrote

Usually the battery or another power source maintaining the magnetic field. Otherwise, the back current induced by the object being picked up would cancel everything out

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DamionFury t1_j8j6cgr wrote

Ah yes. That's right. I also remember we talked about the case of permanent magnets and objects suspended above them. I don't remember what his answer was and I wouldn't be surprised if he told me to try to work it out for myself because my questions were holding the class up.

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kkngs t1_j8l4jhj wrote

If the object is just being suspended by the permanent magnet, being held still against the force of gravity, then no work is being done. It’s not conceptually different than being held up by a shelf.

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agoodpapa t1_j8jpira wrote

Not sure. The ping pong balls have been moved and therefore have absorbed energy.

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KillerCodeMonky t1_j8jr7mn wrote

Movement perpendicular to and away from the earth will convert to potential gravitational energy, which is then released when the ball moves back towards the earth.

In addition, you're assuming that the balls are passive participants. In actuality, from the waves perspective, they are actively resisting being lifted, and actively attempting to fall, limited by their buoyancy in the water. When the wave causes the water to fall away from the balls, they are falling into the water on that trailing edge.

Finally, conservation of energy dictates that that energy does not just disappear. If the energy goes into the ball as movement, something has to stop that movement. That something is going to be either gravity if moving up, or the water itself in any other direction. So the energy is moving from the water, into the ball, back into the water as it resists the ball displacing it to move.

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Monadnok t1_j8ij84r wrote

Someone below pointed out the gravity analogy. Another is a spring, where an object can do work on the spring, and then the spring can return the energy by doing work on the object.

As Feynmann's lecture points out, the electrons that are worked by the applied electric field of the photon can be considered to be "fastened" to the atoms of the materials by a "spring". So the incoming photon moves the electron against a "spring", and the spring returns the energy by working on the electron, which produces a new oscillating electric field to add to the field of the incoming photon.

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Dirty_Virmling t1_j8inomo wrote

Worth pointing out that electromagnetic waves do lose energy when traveling through naturally occurring materials.

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SemiDirectInsult t1_j8mqdsc wrote

How else would radiation raise the temperature of objects? Unless I’m fundamentally misunderstanding something, that’s energy transmission.

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leftoutoctopus t1_j8i04ax wrote

Well, passing through it. I mean, is anything really "free"? Changing the energy from one form (wave) to another doesn't "spend" part of itself?

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KillerCodeMonky t1_j8i2a4l wrote

Well, let's look at a wave in water. It moves some water around, but in a cyclical pattern that should result in very little actual work being done. The same way that lifting something gives you all the energy back when you drop it.

Maybe you're thinking of this from the perspective of a solid object moving through a medium, where it must spend energy on displacing the medium in order to move? This is where my note regarding the wave being energy is important. It does not have to displace anything to move through the medium, the same way that the heat from your stove doesn't displace your pan in order to heat your food. The energy simply goes through.

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DuskyDay t1_j8iio35 wrote

This can't be understood without math.

The wave doesn't lose energy as long as the crystal lattice is perfect, because the physics of our universe allows it to spread in a special way. (The photons don't interact with the particles the medium is made of.)

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skyler_on_the_moon t1_j8iwbez wrote

I watched that whole video to see the ping pong ball experiment and am now disappointed.

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common_sensei t1_j8j1l8a wrote

Try it yourself!... although it probably won't work as described.

The wave would mostly regain the speed after the ping pong ball area, but it would be much reduced in height. A lot of energy would be lost to ripples going everywhere. My analogy just serves to illustrate the general concept of particles reacting to the approach and passing of the wave and generating interfering waves of their own.

However, a similar effect can be seen in wave tanks when you change the depth of the water: https://youtu.be/4_VejGC0DMM?t=261

In this case the interfering waves are bouncing from the new lower bottom of the tank, slowing down the wave.

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TruthOrFacts t1_j8i3b2i wrote

I don't see how this explanation is internally consistent. It is a behavior we see in other types of waves, but in those other cases the wave only appears to slow, and the actual wave proceeds at full speed.

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bandti45 t1_j8ia88q wrote

One thing to remember with photons is that while speed is consistent, intensity isn't.

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RailRuler t1_j8ixshe wrote

Intensity is a measurement of the number of photons arriving times the energy of each photo. The question is asking about a single photon, which is actually meaningful to speak of. Are you saying that the single photon's energy (wavelength/frequency) is changing?

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bandti45 t1_j8jc812 wrote

Well, the person I was replying to was talking about light in general, not a single photon. This is something I only have basic knowledge about.

Interactions with stuff does change it in one way or another. The way the sky is blue is light interacting with air, and tinted glass changes the light going through it. I don't know the mechanics, but that's a change in the photons without changing total speed.

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RailRuler t1_j9kjypo wrote

Those are both the medium filtering out certain colors of light, reducing its intensity. It doesn't change the photons.

The only thing that changes photons is https://en.wikipedia.org/wiki/Fluorescence

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bandti45 t1_j9l737f wrote

I feel like I should have gotten this concept a while ago... thank you for informing me.

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