AAMC 4 Question 15 - Doppler Effect

[AlwaysLucky]

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Question: An astronomer observes a hydrogen line in the spectrum of a star. The wavelength of hydrogen in the laboratory is 6.563 x 10^-7 m, but the wavelength in the star's light is measured at 6.56186 x 10^-7 m. Which of the following explains this discrepancy?

Answer: The star is approaching the earth.

I understand that the wavelength detected from the star is smaller than the wavelength measured in the laboratory. But now what? I have no idea what the answer explanation is saying...

 

[NonTraditional3]

What are the answer choices?

 

[AlwaysLucky]

What are the answer choices?

A) The star is moving away from earth
B) The wavelength of light that the star is emitting changes frequently
C) The frequency of light that the star is emitting changes frequently
D) The star is approaching earth

 

[Jengreef]

If you imagine a ripple effect coming from the star, as the star is approaching earth, these wavelengths that are facing the star are decreasing as it moves. This is essentially a Doppler shift question using light instead of sound.

http://upload.wikimedia.org/wikipedia/commons/c/c9/Dopplereffectsourcemovingrightatmach0.7.gif

The picture above is pretty much what the star is emitting to the observer.

So if you see wavelengths decrease (and the corresponding frequency increase), then you're looking at a Doppler question, and the source of the sound or light or what have you is moving to you as wavelength decreases and moving away from you as wavelength increases.

 

[AlwaysLucky]

If you imagine a ripple effect coming from the star, as the star is approaching earth, these wavelengths that are facing the star are decreasing as it moves. This is essentially a Doppler shift question using light instead of sound.

http://upload.wikimedia.org/wikipedia/commons/c/c9/Dopplereffectsourcemovingrightatmach0.7.gif

The picture above is pretty much what the star is emitting to the observer.

So if you see wavelengths decrease (and the corresponding frequency increase), then you're looking at a Doppler question, and the source of the sound or light or what have you is moving to you as wavelength decreases and moving away from you as wavelength increases.

Ahh, I see .. thx for the diagram link too, it helped!

 

[Jengreef]

no prob. 🙂