Author Topic: 2 questions about light and spectroscopy  (Read 1722 times)

Offline Amy

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2 questions about light and spectroscopy
« on: May 29, 2014, 01:25:51 PM »
So 2 questions I've thought of here, one is related to the episode of Ti-Phy about why the sky is blue.  Why does the sub appear yellow?  Is it just that yellow is the next most common color we are sensitive to see after the blue is scattered in the atmosphere?

The second question is about the effect of red shift on spectroscopy, mainly is it a problem? Is the effect linear among different frequencies of light?  Is it difficult to tell the difference between a star's actual spectrum and the red shift from the acceleration of the expansion of the universe+

Offline bn

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Re: 2 questions about light and spectroscopy
« Reply #1 on: June 01, 2014, 06:47:56 PM »
1. look at a color wheel.
it's not blue specifically which is scattered, but the bluer-purpler end of the spectrum is scattered preferentially.
so what's left when you take all the purple light out of white? you get yellow light.

2. redshifting affects all frequencies by the same amount, so it shifts the true spectrum down by a certain percentage... so you just figure out how much you need to multiply the spectrum you observe... to unsquish it. :p
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Offline scikopas

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Re: 2 questions about light and spectroscopy
« Reply #2 on: June 02, 2014, 11:54:40 AM »

2. redshifting affects all frequencies by the same amount, so it shifts the true spectrum down by a certain percentage... so you just figure out how much you need to multiply the spectrum you observe... to unsquish it. :p

Additionally you can rediscover redshift by making an assumption about what something is that you're looking at.  eg if we know Hydrogen and Helium give a specific emission spectrum (we do) and we can assume a particular redshifted star is made entirely of those (okay its more complicated than that but this is a simplification) then you can say "oh this is the same pattern as hydrogen+helium, but its just shifted down in energy a bit."   

it would mess you up if every element didn't have a distinct pattern, but thankfully these patterns are pretty discrete and not too hard to distinguish.  By this I mean a redshifted helium is never going to look like lithium or any other element and trick us, because--even if the light is shifted--its still a completely different pattern.   physics is neat like that.

PhD student in Materials Science at Arizona State University currently working on high-temperature superconductors and quantum computers or something.
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