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# redshift formula: z = change in wavelength / original wavelength (need clarification) Watch

1. lets say wavelength = l and change in wavelength dl

z = dl/l, where l = original wavelength

Light from a distant galaxy has an emission line with wavelength 445nm. The line has wavelength 410nm when measured in the lab. Calculate the galaxy's redshift

Ans: 35/310

i thought it would be z = 35/445? why is 410nm the original wavelength?

any help appreciated, thanks in advance
2. (Original post by the maths guy)
lets say wavelength = l and change in wavelength dl

z = dl/l, where l = original wavelength

Light from a distant galaxy has an emission line with wavelength 445nm. The line has wavelength 410nm when measured in the lab. Calculate the galaxy's redshift

Ans: 35/310

i thought it would be z = 35/445? why is 410nm the original wavelength?

any help appreciated, thanks in advance
Original wavelength means the wavelength in the frame it was emitted, i.e. measured when there is no relative motion between the source and the observer. Given we are trying to infer the relative motion of the galaxy, it makes much more sense to define it this way.

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Updated: December 8, 2010
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