wave speed and medium confusion

[lamborghiniMD]

Okay so I get that the wavelength increases as it goes from let's say air to water, but I found this concept confusing when dealing with index of refractions.

Because Snell's law says that as a wave passes from a lower index to a higher index of refractions, the wavelength decreases and bends toward the normal.

So how do we know when to apply these concepts? I mean I guess I know, but I can see how the MCAT writes might make this confusing on a real test. Any thoughts?

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[cloak25]

n=c/v
v=wavelength*f

higher index of refraction = lower speed = bend towards normal.

from v = wavelength*f, assuming f is constant, so since v decreased, wavelength must also decreased

 

[lamborghiniMD]

Yes I know that, but what Im trying to say is that in a passage or a discrete question, are we supposed to know which trend to use based on the info and context clues? What if the question is so vague that you don't know which trend to follow? Perhaps I'm being paranoid, but better safe than sorry.

 

[lamborghiniMD]

Wait I'm confused by your response. It is a known fact that wavelengths increases as it goes from gas to liquid to solid?

 

[cloak25]

I had a similar question on a different problem. Maybe this thread can help

http://forums.studentdoctor.net/showthread.php?t=926743

 

[Jengreef]

Okay so I get that the wavelength increases as it goes from let's say air to water...

Does it? I've always known for wavelength to decrease as it's going form one medium to a more dense medium.

Light slows down in the above scenario and sound speeds up in the above scenario. Can anyone confirm this?

 

[bk9iq]

Yes light slows down while sound speeds up

 

[Fivo]

Wait I'm confused by your response. It is a known fact that wavelengths increases as it goes from gas to liquid to solid?

Yes.

v = hf (wavelength x frequency)

For instance, a sound wave moves faster through a solid compared to a gas, so that means "v" increases (since velocity (v) depends on the medium).
Frequency depends on the source and NOT the medium, so that means "f" will remain constant.
Now since "v" is increasing and "f' is constant, then that means "h" will have to increase.

 

[alamo4]

Does it? I've always known for wavelength to decrease as it's going form one medium to a more dense medium.

Light slows down in the above scenario and sound speeds up in the above scenario. Can anyone confirm this?

Maybe it helps if think about sound being transmitted by the medium itself, but think of light more as it's particle like form traveling through material.

In general, more rigid or more elastic materials transmit pressure waves much more efficiently. If you bang on a metal rod, the vibration travels quickly down the metal. Slapping water is less efficient, and slapping the air is even worse. Intuitively, slaps and vibrations in the metal rod should be transmitted with the greatest speed.

When a photon enters material, it has to deal with atoms getting in its way or deflecting it slightly, etc. The more dense the material and the more the atoms of material interfere (air, water, plastic, silver, lead), the more difficult it is and the slower it has to go, until it is finally reflected or absorbed instead of transmitted. When it comes to an interface between two different materials at an angle, you might think about it being like a car suddenly getting some traction under the wheels on one side, going a little faster in that direction until all the wheels are on the new surface and it can really "peal out" in the new straight path which is at a little bit of an angle from the original direction of travel. Not the best analogy, but may help you get the right intuition, so you'll know which direction the light bends.

 

[Jengreef]

So would it be safe to say sound wavelength increases as it's traveling from a less dense medium to a more dense medium, and that light wavelength decreases in the same scenario?

 

[alamo4]

For light density doesn't exactly apply, that was more of analogy, think refractive index, but that's the idea.

Speed = frequency * wavelength

The frequency is fixed, so if the speed in the medium goes up, then wavelength goes up too, and vice versa.

Anyway, light gets shorter wavelengths going from air to water, because it is going slower. Sound gets longer wavelengths going from air into water, because it speeds up by a factor of about 5, I think.

The frequency is fixed, or the waves would somehow "bunch up" or something weird like that which is hard to imagine, but it's not something that happens.

 

[Jengreef]

Right, from what I understand, frequency only changes for Doppler shifts and nothing else. Well, if nothing tested on the MCAT anyway.

 

[lamborghiniMD]

Um yes it is tested on the mcat lol, it was on mine last year that's why I wanna clarify lol

 

[Jengreef]

Ah, you misunderstood my earlier post. I meant outside of Doppler shifts, frequency doesn't change within the scope of the MCAT. Might be that frequency never changes outside of Doppler shifts, but I feel like someone would prove me wrong immediately after making such a statement, lol.

 

[Sandstorm]

So would it be safe to say sound wavelength increases as it's traveling from a less dense medium to a more dense medium, and that light wavelength decreases in the same scenario?

Correct

 

[Sandstorm]

Ah, you misunderstood my earlier post. I meant outside of Doppler shifts, frequency doesn't change within the scope of the MCAT. Might be that frequency never changes outside of Doppler shifts, but I feel like someone would prove me wrong immediately after making such a statement, lol.

Actual frequency doesn't change on the mcat ever...in the case of doppler your perceived frequency changes, not actual.