Doppler Effect

[ItsJason]

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Hi guys,

I have a question about an AAMC doppler effect question, it states:

An astronomer observes a hydrogen line in the spectrum of a star. The wavelength of hydrogen in the laboratory is 6.563e^-7m, but the wavelength in the star's light is measured at 6.56186e-7. Which of the following explains this discrepancy?
A. The star is moving away from Earth
D. The star is approaching Earth

I choose A because the observed frequency should be considered lower than the emitted frequency, but the answer is D, the star is approaching Earth. And by doppler's equation of:

f (observed) = f(source) ((V +/- Vo)/(V +/- Vs))

it should indicate that the source (star) is moving away from the observer.

Thanks! 🙂

 

[MedHopeful20134]

@ItsJason

Be careful! The question indicates that the wavelength decreases (not frequency).

 

[ItsJason]

@ItsJason

Be careful! The question indicates that the wavelength decreases (not frequency).

I think I jumped a bit too much in my reasoning, sorry! I meant to state that because the laboratory had a wavelength longer than the wavelength of the star's emitting light, that it has a smaller frequency. This is concluded because of v = (lambda)(frequency), where the velocity would have remained constant.

 

[justadream]

@ItsJason

"I meant to state that because the laboratory had a wavelength longer than the wavelength of the star's emitting light, that it has a smaller frequency."

Not sure what you mean by that.

The point is that the wavelength detected by the lab is smaller than the wavelength emitted by the star. Since velocity is constant, if wavelength detected is smaller, frequency detected must be higher..

This occurs when the source (the star) and the detector (the lab) are approaching each other.

Btw: You should post these in the Official AAMC threads (http://forums.studentdoctor.net/thr...q-a-discussion-threads-rules-of-order.516243/) or your thread will probably be locked.

 

[ayocaptain628]

Hi guys,

I have a question about an AAMC doppler effect question, it states:

An astronomer observes a hydrogen line in the spectrum of a star. The wavelength of hydrogen in the laboratory is 6.563e^-7m, but the wavelength in the star's light is measured at 6.56186e-7. Which of the following explains this discrepancy?
A. The star is moving away from Earth
D. The star is approaching Earth

I choose A because the observed frequency should be considered lower than the emitted frequency, but the answer is D, the star is approaching Earth. And by doppler's equation of:

f (observed) = f(source) ((V +/- Vo)/(V +/- Vs))

it should indicate that the source (star) is moving away from the observer.

Thanks! 🙂

so what this question is saying is:
wavelength of star's light lower than wavelength of hydrogen line
what causes this diff?

if lambda*f = v
where v is constant and lambda is decreasing for the star, that means the f is bigger
conceptually we know that frequency increases as either the star or the object or both move towards each other .:. D

while this is the doppler effect formula:
f (observed) = f(source) ((V +/- Vo)/(V +/- Vs))

try not to get bogged down by that and just remember
if they are moving closer to each other, f' is greater
and if they move away from each other, f' is smaller

 

[ItsJason]

I understand that everyone is saying that the frequency would be higher for the star because it does have a smaller wavelength, but it is the SOURCE of the light, and by the Doppler equation, the detected frequency thereby lowered (in the laboratory) because the wavelength is higher.

Isn't it all in a matter of perspective? If I think of it in terms that the laboratory is the source, then the "detected" frequency will be from the star and it is higher, so therefore they are approaching (as the equation shows); however, in perspective of the star being the source (in which case I think this is), the detected frequency ends up being lower and therefore shows that they are moving away.

Can someone advise in terms with the equation ? 🙂 I saw another thread post about this and they just stated that the H-line was a standard on earth rather than what was emitted by light, but I feel that's too ambiguous for the question.

 

[justadream]

@ItsJason

I don't even know how you would use the equation here because the relative velocity of the detector and the source is not provided (and you don't need it).

f ' is the detected frequency.
V0 is the speed of the detector
Vs is the speed of the source
f0 is the source frequency
V = c = 3x10^8

If the star and source are moving toward each other, then you either increase the numerator (v PLUS something) or decrease the denominator (v MINUS something).

The star is the source here so you should do v MINUS something in the denominator. The detected frequency will be higher.

If you, for some reason, wish to take the lab as the source, then you would do v PLUS something in the numerator. Here again, the detected frequency will he higher. I'm pretty sure that if you flip this around (as in, switch what is the source and what is the detector), the number you calculate for the detected frequency will be different (but in both cases, the detected frequency increases).

But to type everything I did in my last few paragraphs, I had to come up with the conclusion I first bolded above. In other words, there is no reason to use the equation here.

 

[ItsJason]

@justadream

Yes, I understand how the detected frequency would change in terms of the equation relative to the source and the detector, but when I was "calculating" it, and how I concluded shows that they are moving away from each other.

For example:
Wavelength of laboratory > Wavelength of Source (Star)
Therefore, Frequency of laboratory < Frequency of Source because of lambda*f = v (where v = c)
Then, if the V = c, and we know that f' < fo, the only way to experience this change is that either the velocity of the detector is MINUS, or the denominator is PLUS.

Sorry about me not really understanding everyone's idea, but because I keep seeing it like this causes me to be stumped. I should've been more specific earlier, sorry!

 

[justadream]

@ItsJason

After reading the question about 50x, I got a bit confused myself.

I think the confusion is the wording:

In the question, the
DETECTOR =6.56186e-7 (this is what is MEASURED)
SOURCE = 6.563e^-7 (this is confusing because it's done in the LAB)

Do you agree with these assignments? I think you have it backwards.

Since the wavelength of detector < wavelength of source, frequency of detector > frequency of source.

 

[ItsJason]

@justadream

That's the exact wording of the question itself. I don't understand how they would indicate something done in the lab as the source when the star is the source of the hydrogen line. Other than this, I can connect frequency/wavelength, but I guess it's just defining which is detector and source.

 

[justadream]

@ItsJason

The thing done in the lab is just to see what the wavelength of the light being emitted form the theoretically is (aka what the SOURCE is).

You'll notice that they use the word MEASURED when they mention the wavelength associated with the star. This is why that is the DETECTOR.

Think about it. If you didn't assign the experiment done in the lab as the SOURCE, how would you ever get the source frequency (you can't! - if you tried to measure it, it would be the DETECTED).

 

[ItsJason]

@justadream

That makes complete sense! My goodness, the wording of the question will always get me on these... it's like they want to test verbal in physical sciences!

Thanks for all the help, I was completely stumped on how they would choose either as the detector/source, but your explanation clears it all up.