Waves

[chiddler]

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The following statement is false.

The energy carried by a wave cannot be distinguished from the energy carried by a particle.

Besides plain sounding wrong, I don't understand why this is false very well. Would really appreciate help.

Thank you!

also what is coherence in the context of waves?

"Coherence is the degree to which the difference in phase between two or more waves is constant over some region of time or space." Do not understand :x

ONE MORE: "Any type of wave...can interfere with another wave." Sound and light? Really really?

 

[milski]

On the false statement: I don't like how it's phrased at all. Energy is energy - you cannot tell how it was carried to whatever its state is at the moment of observation. In other words, if you have hot water, you cannot tell if it is hot because you were microwaving it or because you dropped a block of hot iron in it. I would guess that they are trying to talk about carrying energy by particles involves transfer of mass while that's not the case with waves - they do pretty good job of transferring energy even through vacuum. There is also the whole particle/wave duality thing, but I don't think we should have to go there.

Coherence is usually discussed in the context of interference of two waves. Think about trying to match a specific phase of one wave to a specific phase of the other wave. For example, let's have two waves with the same frequency. The peaks of one of them will always match the same place on the other wave - be it another peak, or the equilibrium position or any other place. The important part is that when the first wave is at its peak, the second one will always be in the same phase. These two waves are completely coherent.

Now consider two waves with a very slight difference in their frequencies. If you look at what phase of wave 2 matches the peaks of wave 1, you'll see that it very slowly changes from peak to peak. These waves are coherent to a lesser degree.

 

[milski]

ONE MORE: "Any type of wave...can interfere with another wave." Sound and light? Really really?

What types do they have in mind? You could make the case for interference between sound and light but that's unlikely to be measurable or practical. Any chance that they are talking about harmonic vs non-harmonic or any other sort of classification?

 

[MedPR]

Stop posting physics questions. You're making me feel dumb because physics is so freaking hard for me.

 

[milski]

Stop posting physics questions. You're making me feel dumb because physics is so freaking hard for me.

But then I'll have to read bio questions and wonder what are you talking about and will we ever cover that in class. 😛

 

[MedPR]

But then I'll have to read bio questions and wonder what are you talking about and will we ever cover that in class. 😛

Let's just do chem questions, I'm good at those 🙂

 

[chiddler]

What types do they have in mind? You could make the case for interference between sound and light but that's unlikely to be measurable or practical. Any chance that they are talking about harmonic vs non-harmonic or any other sort of classification?

The question: #42 berkeley review
Waves interfere when they superpose. No interference effects are observed between light beams emitted from two different flashlights or between sound waves emitted from two violins. This means:
A. only waves on strings exhibit interference.
B. the interference process happens in such cases but the difference between the superposed wave and the component waves is too slight to detect.
C. light is actually a particle and it therefore cannot show interference.
D. the interference pattern process does happen in such cases, but the incoherence of the wave sources blurs out any interference effects.

The answer: Any type of wave, regardless of wavelength or phase, can interfere with another wave. then they give an example of the double slit experiment.

 

[chiddler]

I"M SORRY MEDPR D:!

so if two waves are composed of two identical frequences then they are perfectly coherent. when more different frequences enter the picture, then it becomes more and more incoherent. so it's a sliding scale and the only time a wave is "coherent" is if it is composed of identical frequencies.

didn't understand the false statement explanation either. but i'm not too worried about that if you don't feel like elaborating.

 

[milski]

I"M SORRY MEDPR D:!

so if two waves are composed of two identical frequences then they are perfectly coherent. when more different frequences enter the picture, then it becomes more and more incoherent. so it's a sliding scale and the only time a wave is "coherent" is if it is composed of identical frequencies.

didn't understand the false statement explanation either. but i'm not too worried about that if you don't feel like elaborating.

Yes, two harmonics at the same frequency would be the most coherent case. A lot of times you would consider coherence for non-harmonic signals so there is a lot more variety in how coherent two signals can be besides having similar/different frequencies. But for MCAT purposes I can't imagine having to know anything further than same freq = coherent.

I'm not sure myself what they are trying to say there. One possibility is that in the case where the particle carries the energy, the particle moves with it (think courier & package) while in the case of a wave it's more like a relay - the energy moves but the particles stay at the same place. That works well on macro level (air, sound, water waves, etc) but I would not push it further to things like light.

For question #42: They are asking you to compare flash light to flash light and violin to violin, not between violin and flashlight. In the explanation they are trying to say that the interference happens between waves with different frequencies too but since both the flashlights and the violins are not coherent sources, you cannot observe any interference patterns. If you had a laser (which is a coherent source) the patterns are there and fairly easy to observer.

The 'any type' language is a bit sloppy here, I agree, but what they're trying to get to is that all types of waves are subject to interference (with other waves from the same type).

 

[chiddler]

thank you very much for your help.