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

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Discussion Overview

The discussion focuses on how to calculate the wavelength of light coming from an object located one light year away. Participants explore the implications of distance on light's wavelength, including factors such as motion and redshift.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant seeks a formula for calculating the wavelength of light from a distance of one light year.
  • Another participant clarifies that "light" can refer to any frequency of electromagnetic radiation and discusses how proper motion can affect observed wavelength through red or blue shifting.
  • A participant mentions the Hubble expansion formula and its limitations, stating it applies primarily to galaxies and not to nearby stars due to gravitational binding.
  • There is a discussion about the lack of expansion effects at the scale of one light year, emphasizing that relative motion within a galaxy is the primary source of any Doppler shift.
  • Some participants express that the context of the question may not align with the complexities of cosmic expansion and redshift.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of cosmic expansion to the calculation of wavelength at one light year, with some asserting that it is not applicable at this scale. The discussion remains unresolved regarding the best approach to calculate the wavelength in this context.

Contextual Notes

There are limitations in the assumptions made about the motion of the light source and the observer, as well as the definitions of terms like "light" and "galaxy." The discussion does not resolve how these factors influence the calculation of wavelength.

shantanu23
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TL;DR
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.

Please help.
 
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shantanu23 said:
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.

Please help.
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.
 
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.
 
sophiecentaur said:
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.
 
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Ibix said:
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.
Which is why I responded the way I did, given that he was talking about only a 1 LY distance.
 
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