AAMC 9CBT #47 (Physics)

[MedPR]

Visible light travels more slowly through an optically dense medium than through a vacuum. A possible explanation for this could be that the light:

A. is absorbed and re-emitted by the atomic structure of the optically dense medium.
B. loses amplitude as it passes through the optically dense medium.

Answer is: A


The explanation is that both answers are "true" but only one (in white) directly answers the question. I don't understand how the wrong answer doesn't also directly answer the question.

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[MedPwnage]

amplitude isn't speed

 

[aspiringdoc09]

Visible light travels more slowly through an optically dense medium than through a vacuum. A possible explanation for this could be that the light:

A. is absorbed and re-emitted by the atomic structure of the optically dense medium.
B. loses amplitude as it passes through the optically dense medium.

Answer is: A


The explanation is that both answers are "true" but only one (in white) directly answers the question. I don't understand how the wrong answer doesn't also directly answer the question.

Amplitude is not affected by velocity, frequency, and wavelength. Likewise, according to the wave speed formula, amplitude doesn't affect speed. Also, you know already that the fastest sound/light can travel is at the speed of light and it only travels at this max speed when in a vacuum. Any deviation from a vacuum and the speed decreases as per the formula for the index of refraction (n=c/v). Answer A also throws little Gen Chem in there because the molecules will absorb the light and re-emit which will slow the speed that the light passes through the medium.

 

[MedPR]

amplitude isn't speed

Amplitude is not affected by velocity, frequency, and wavelength. Likewise, according to the wave speed formula, amplitude doesn't affect speed. Also, you know already that the fastest sound/light can travel is at the speed of light and it only travels at this max speed when in a vacuum. Any deviation from a vacuum and the speed decreases as per the formula for the index of refraction (n=c/v). Answer A also throws little Gen Chem in there because the molecules will absorb the light and re-emit which will slow the speed that the light passes through the medium.

But when the wavelength lengthens, doesn't the amplitude of the wave decrease?

 

[milski]

But when the wavelength lengthens, doesn't the amplitude of the wave decrease?

For an already existing wave - no. The only way to decrease the amplitude would be some sort of energy loss in the media.

I am saying "existing wave" since you can change a lot of things when creating the wave.

 

[MedPR]

For an already existing wave - no. The only way to decrease the amplitude would be some sort of energy loss in the media.

I am saying "existing wave" since you can change a lot of things when creating the wave.

I think I had a problem understanding this a month ago as well. So how does the wavelength increase without the amplitude decreasing?

 

[milski]

I think I had a problem understanding this a month ago as well. So how does the wavelength increase without the amplitude decreasing?

Think about a spring, looking at it from the side. The diameter is the amplitude, the distance between each loop - the wavelength. If you stretch the spring, the amplitude stays the same but the wavelength increases.


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[MedPR]

Think about a spring, looking at it from the side. The diameter is the amplitude, the distance between each loop - the wavelength. If you stretch the spring, the amplitude stays the same but the wavelength increases.


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Oh I see, thanks!

 

[aspiringdoc09]

I think I had a problem understanding this a month ago as well. So how does the wavelength increase without the amplitude decreasing?

By changing the frequency according to the wave formula.

 

[MedPR]

Re-reviewing today, AAMC says that the wave does lose amplitude as it passes through optically dense medium. Unless I'm misunderstanding, everyone in here is saying that the amplitude doesn't change..

 

[milski]

It's like friction. In the ideal case, moving through the more optically dense media does not change the amplitude. In reality, there are losses and the amplitude will eventually decrease.

The losses are in intensity, not speed.

 

[MedPR]

It's like friction. In the ideal case, moving through the more optically dense media does not change the amplitude. In reality, there are losses and the amplitude will eventually decrease.

The losses are in intensity, not speed.

Well then I'm back to the beginning. Earlier you said that amplitude of an existing wave will only decrease if there is energy loss. Energy is defined by E=hc/lambda right?. So if the only way to decrease amplitude is to lose energy, and a loss in energy results in a gain in wavelength, then why doesn't wave speed go up since c=f*lambda and frequency is constant since it is determined by the source.

Is it because E=h*f so if energy is lost, then frequency goes down (despite typically being controlled by the source...) and the decrease in frequency is the exact same in magnitude as the increase in wavelength, so they cancel each other out and the wave speed (v=f*lambda) stays the same?

If that's the case, in what cases does "frequency is determined by the source" apply?

 

[milski]

The energy loss will not happen for a single photon. As the photons move, some of them hit electrons and move them to a higher energy state. Eventually, the electrons fall back to their initial state and the photons are being retransmitted. The retransmitted photon will have exactly the same energy as the initial one - energy can be transferred only in discrete amounts. Not all of the electrons will return to ground state - some of them will stay longer at the higher energy state. These will result in "loss" of photons and decreased amplitude/intensity.

You cannot change c from from c=fλ. It is a constant here and determines how f and λ are related. Frequency is whatever it was at the source - it cannot change on its own. For mechanical waves, that's just how they work. For a photon, that's it's energy and it cannot transfer only parts of it. The only thing that can change when moving to a media with different c is λ and that's what happens.

 

[MedPR]

So intensity/amplitude are unrelated to the speed of the wave and are mostly used when considering what the wave looks like in a medium? So even if the amplitude looks smaller (or is smaller) because there are less particles being displaced, the speed of the wave is still the same?

 

[milski]

So intensity/amplitude are unrelated to the speed of the wave and are mostly used when considering what the wave looks like in a medium? So even if the amplitude looks smaller (or is smaller) because there are less particles being displaced, the speed of the wave is still the same?

Yes. Intensity/amplitude is related to how many photons are "left in the race." The speed in that specific media is fixed - they all run at the same speed.

 

[MedPR]

Yes. Intensity/amplitude is related to how many photons are "left in the race." The speed in that specific media is fixed - they all run at the same speed.

Ok but photons that run into and excite electrons will be back in the race once the electron relaxes and the photon is re-emitted, right?

 

[milski]

Ok but photons that run into and excite electrons will be back in the race once the electron relaxes and the photon is re-emitted, right?

Eventually, yes. And they will be back running at the same speed and frequency - they have no other options.

 

[MedPR]

Eventually, yes. And they will be back running at the same speed and frequency - they have no other options.

Thank you, that is a really really great analogy!

 

[wannabedoc0209]

Thank you for great explanations & analogies above, but I'm still a little confused (sorry!): If a decrease in amplitude results in energy loss, then wouldn't that imply that speed & wavelength change? That's contradictory to the above "amplitude is not affected by velocity, frequency, and wavelength" statement, in my head at least, and I'd appreciate any input to help me solve this

 

[humblethinker]

Thank you for great explanations & analogies above, but I'm still a little confused (sorry!): If a decrease in amplitude results in energy loss, then wouldn't that imply that speed & wavelength change? That's contradictory to the above "amplitude is not affected by velocity, frequency, and wavelength" statement, in my head at least, and I'd appreciate any input to help me solve this

the old version of the exam focused heavily on using the velocity=frequency x wavelength formula. Understanding that formula is the information to draw upon and interpret in order to correctly answer this question.

 

[BerkReviewTeach]

Thank you for great explanations & analogies above, but I'm still a little confused (sorry!): If a decrease in amplitude results in energy loss, then wouldn't that imply that speed & wavelength change? That's contradictory to the above "amplitude is not affected by velocity, frequency, and wavelength" statement, in my head at least, and I'd appreciate any input to help me solve this

I think it might be best to disregard the fundamental wave equation and visualize the movement of light through a medium in general.

Amplitude is in essence the number of photons within the light packet. The question is asking about speed, and what is causing the light packet to slow down (take a longer time to traverse the path.) If light is absorbed along the path, then its intensity (amplitude) will decrease. If light is absorbed and then re-emitted, then its intensity (amplitude) should remain the same. However, time spent being absorbed and then re-emitted is time not being spent moving across the path, so it takes longer for that packet of light (group of photons) to traverse that path, meaning it is moving slower on average.

The important thing here is that rather than getting caught up in distractors and extraneous information, zero in on one key concept in the question: speed. The speed is reduced when passing through the medium because it is absorbed, held for a brief amount of time, and then re-emitted by the atoms of the medium (which store it temporarily by exciting an electron.) The amplitude is not related to the speed, so choice D is extraneous information, inserted to throw you off the scent. The fact that the amplitude drops (obeying to Beer's law), is irrelevant to the decrease in speed.

Consider this weird analogy. When you roll down your car window while driving, your hair gets messed up and it becomes harder to hear.

Question: A driver's hair gets into a disarray when exposed to fast moving air associated with a vehicle moving at high speed with the windows lowered. A possible explanation for this could be that:

A. the hair is jostled by collisions of air molecules with strands of hair .
B. the ambient decibel level increases due to more collisions with the eardrum.

Choice B is a completely true, albeit irrelevant statement. Only choice A is a valid explanation.