# How to calculate the wavelength of light coming from 1 light year away

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## Summary:

How to calculate the wavelength of light coming from 1 light year away.
Is there any formula for it.

## Main Question or Discussion Point

How to calculate the wavelength of light coming from 1 light year away.
Is there any formula for it.

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phinds
Gold Member
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Summary:: How to calculate the wavelength of light coming from 1 light year away.
Is there any formula for it.

How to calculate the wavelength of light coming from 1 light year away.
Is there any formula for it.

I think you have a fundamental misunderstanding of light. "Light" is an English language term which is used either for (1) any frequency of electromagnetic radiation or for (2) that range of the electromagnetic spectrum which is in the visible by humans.

The wavelengths range from near zero to extremely high. If there is no proper motion between you and the 1 light year distant object, then the wavelength is just a measure of what in English is called the "color". If there IS proper motion then there can be red or blue shifting so that the wavelength you see is not exactly that emitted.

sophiecentaur
Gold Member
Are you talking about the Expansion of the Universe and the red shift here? A quick buzz through this wiki link will tell you what you need to know and the context.
The equation v = H0D tells you only the average speed of recession v for galaxies at distance D, due to the actual expansion of the space. That is only a ballpark figure because there will be motion of Earth relative to each galaxy.
If you measure the frequency of a spectral line in light from a galaxy, you can use the Doppler Formula to obtain v from the frequency shift relative to the spectrum of the elements on Earth.

The Hubble expansion formula doesn't apply to nearby stars - in our own galaxy - because they are constrained by the local gravitational field within the galaxy. H0 can only be used for galaxies because they are considered 'independent' of the mass of our galaxy. But you can tell the relative motion between a star and Earth by measuring the frequency shift (red or blue, depending)

It's down to what you actually describe as a galaxy (see this link and others) but the sort of distance between us and the nearest dense groups of stars is tens of thousands of light years and small galaxies in our local cluster are much further away than that. Hubble definitely can't tell you anything about the likely frequency shift at 1LY.

Ibix
Hubble definitely can't tell you anything about the likely frequency shift at 1LY.
More than that, there is no effect from the expansion of the universe at this scale. Stars in a galaxy (or even the local group of galaxies) are gravitationally bound and are not receding due from us as more distant objects are. The relative motion of the star in its path around the galaxy is the only source of a Doppler shift.

• sophiecentaur
phinds
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