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# Problems with redshift

Physics and electronics discussion, revision, exam and homework help.

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1. Problems with redshift
can anyone please help me interpret the absorption spectrum that shows the redshift? I mean the dark lines move towards the red end of the spectrum but how does it show that the object emitting the spectrum is moving away from us and making longer wavelengths behind it? And what are the dark lines anyway..?
2. Re: Problems with redshift
The dark lines represent the light frequencies which are absorbed by the object, everything else is reflected. You can use the dark lines to interpret which elements are in the object.

If an object is moving away from you the emitted waves will have slightly elongated wavelengths corresponding to those dark lines. So every other emitted light is red shifted, and therefore the dark lines also move towards the red end.

Look up the Dopper Effect as to why this happens.
3. Re: Problems with redshift
remember, in the visible spectrum: red has the longest wavelength, and violet has the shortest wavelength...
so if the lines are shifted towards the red, you know their wavelengths have become longer --- does that make more sense?

and, like the guy above said, look up the doppler effect if you want to know why the wavelength gets longer but i'm assuming you're gcse, in which case you probably don't need to know why
4. Re: Problems with redshift
(Original post by Nice.Guy)
remember, in the visible spectrum: red has the longest wavelength, and violet has the shortest wavelength...
so if the lines are shifted towards the red, you know their wavelengths have become longer --- does that make more sense?

and, like the guy above said, look up the doppler effect if you want to know why the wavelength gets longer but i'm assuming you're gcse, in which case you probably don't need to know why
No man, i just started A levels..It's been five months...I know all about the Doppler effect but having trouble understanding the red shift. So the dark lines represent the absorption of the light waves by particular elements that only absorb waves of certain frequencies and thus wavelengths, right? so if the object emitting the light moves away, then the elements will be shown to absorb waves of different wavelengths or something? I just don't understand this bit. what does the moving away of the dark lines mean?
5. Re: Problems with redshift
yep correct. And good question, i don't think the elements absorb different wavelengths - it's just they APPEAR different when we pick them up here on earth.
http://www.acs.psu.edu/drussell/Demo...r/doppler.html

the diagrams on this website may help ^. Looking at the second diagram, where the dot is moving to the right, imagine yourself to the left of it. Can you see the that waves are reaching you with a LOWER frequency? (fewer waves per second). Because the dot is moving AWAY. That's all red shift is.
lower frequency = LONGER wavelength
so if all the wavelengths picked up are longer, they will all move closer to the red side (red has the longer wavelength). Does that help at all?
6. Re: Problems with redshift
(Original post by Nice.Guy)
yep correct. And good question, i don't think the elements absorb different wavelengths - it's just they APPEAR different when we pick them up here on earth.
http://www.acs.psu.edu/drussell/Demo...r/doppler.html

the diagrams on this website may help ^. Looking at the second diagram, where the dot is moving to the right, imagine yourself to the left of it. Can you see the that waves are reaching you with a LOWER frequency? (fewer waves per second). Because the dot is moving AWAY. That's all red shift is.
lower frequency = LONGER wavelength
so if all the wavelengths picked up are longer, they will all move closer to the red side (red has the longer wavelength). Does that help at all?
Yeah, thanks a bunch...I think i get it now

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Last updated: August 3, 2012
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