Doppler Shift

[IncuSpy]

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I really have a good understanding of doppler shift but this problem is throwing me off. Can you offer me some insight into why I cant see this?

An astronomer observes hydrogen spectrum. The wavelength of hydrogen in the laboratory is 6.563 x 10 -7 but the wavelength in the star's light is measured at 6.56186 x 10 -7. Which explains this?

I put that the star is moving away since I figured, relative to the star, on Earth we observe the wavelength getting bigger which should mean that it is moving away. BUT the correct answer is it is approaching because "the length detected from the star is smaller than the lab value, which is associated with approaching relative velocity."

Can someone clarify this discrepancy?

 

[sv3]

I really have a good understanding of doppler shift but this problem is throwing me off. Can you offer me some insight into why I cant see this?

An astronomer observes hydrogen spectrum. The wavelength of hydrogen in the laboratory is 6.563 x 10 -7 but the wavelength in the star's light is measured at 6.56186 x 10 -7. Which explains this?

I put that the star is moving away since I figured, relative to the star, on Earth we observe the wavelength getting bigger which should mean that it is moving away. BUT the correct answer is it is approaching because "the length detected from the star is smaller than the lab value, which is associated with approaching relative velocity."

Can someone clarify this discrepancy?

The second number is smaller, indicating a smaller wavelength, which must mean its closer to you.

 

[IncuSpy]

I still dont see it. The source (the star) has a smaller wavelength than the observer (earth) therefore it must be moving away - that was my thought.

Or is this not an issue of observer and source? Is it rather that hydrogen's WL should be the value in the lab, and since we observe it (in the star) to be less, that it is getting closer?

 

[G1SG2]

I really have a good understanding of doppler shift but this problem is throwing me off. Can you offer me some insight into why I cant see this?

An astronomer observes hydrogen spectrum. The wavelength of hydrogen in the laboratory is 6.563 x 10 -7 but the wavelength in the star's light is measured at 6.56186 x 10 -7. Which explains this?

I put that the star is moving away since I figured, relative to the star, on Earth we observe the wavelength getting bigger which should mean that it is moving away. BUT the correct answer is it is approaching because "the length detected from the star is smaller than the lab value, which is associated with approaching relative velocity."

Can someone clarify this discrepancy?

There is no discrepancy. Since the measured wavelength was lower than the one in the lab, that means the measured frequency must be higher than the one in the lab. Shorter wavelength=higher frequency=blue shift=star moving toward the earth.

If the measured wavelength was LARGER than the one from the lab, then that would give us a smaller relative frequency. Larger wavelength=smaller frequency=red shift=star moving away from the earth.

But, since the measured wavelength was lower, we have a blue shift, and thus an approaching star. You should know that wavelength and frequency are inversely related.

 

[amine2086]

I really have a good understanding of doppler shift but this problem is throwing me off. Can you offer me some insight into why I cant see this?

An astronomer observes hydrogen spectrum. The wavelength of hydrogen in the laboratory is 6.563 x 10 -7 but the wavelength in the star's light is measured at 6.56186 x 10 -7. Which explains this?

I put that the star is moving away since I figured, relative to the star, on Earth we observe the wavelength getting bigger which should mean that it is moving away. BUT the correct answer is it is approaching because "the length detected from the star is smaller than the lab value, which is associated with approaching relative velocity."

Can someone clarify this discrepancy?

The doppler effect equation you should apply in this case is:
delta(lamda)/(lamda(source)) = relative velocity between source and observer/(wave velocity)

From the numbers given, you can see that delta(lamda) is negative. Since there is a delta(lamda) the star must be moving relative to you (the observer in the lab). Since we have negative delta(lamda), the star must be moving closer to the observer. In other words, the source and the observer are getting closer to each other.

 

[IncuSpy]

ok, thanks everybody

 

[Dr Gerrard]

The doppler effect equation you should apply in this case is:
delta(lamda)/(lamda(source)) = relative velocity between source and observer/(wave velocity)

From the numbers given, you can see that delta(lamda) is negative. Since there is a delta(lamda) the star must be moving relative to you (the observer in the lab). Since we have negative delta(lamda), the star must be moving closer to the observer. In other words, the source and the observer are getting closer to each other.

I don't think that equation works.

delta(frequency)/frequency(source) = relative velocity between source and observer/(wave velocity) as well.

In this case, delta(f) would be positive, so according to that equation, they should be moving away from each other. I know this is not true, but I think this might just show that that equation is used for magnitudes, and does not signify direction.

EDIT: Did I get the frequency equation wrong? Is the right side of the equation negative?

 

[IncuSpy]

After changing the way I thought about it, it isnt really necessary to know the equation. I just had the wrong perspective:

"Or is this not an issue of observer and source? Is it rather that hydrogen's WL should be the value in the lab, and since we observe it (in the star) to be less, that it is getting closer?"

I dont think this problem has an observer. You are just comparing the source to the experimental value.

Thanks for helping me think through this everyone

 

[G1SG2]

After changing the way I thought about it, it isnt really necessary to know the equation. I just had the wrong perspective:

"Or is this not an issue of observer and source? Is it rather that hydrogen's WL should be the value in the lab, and since we observe it (in the star) to be less, that it is getting closer?"

I dont think this problem has an observer. You are just comparing the source to the experimental value.

Thanks for helping me think through this everyone

You need to have an observer/detector for the doppler shift. I think the problem lies in what you perceived to be the actual measurement (lab vs source).