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Imagine a green light source is at the center of a transparent material sphere and I am staying in the vacuum (or air) and looking to it.

Now imagine that the wave length of the this light increases drastically when it goes out of the sphere.

And I would see the source same color whether I am inside the glass or outside the glass. Although wavelengths change while traveling from one medium to another, colors do not, since colors are associated with frequency.

But why are the colors associated with their wavelengths in vacuum instead of their frequency since frequency seems to describe more characteristic of color vision independent of the medium?

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    $\begingroup$ When your eye observes the color of the light, it's travelling in the material of your eye, not the vacuum around you or any other material it traveled through to get to your eye. $\endgroup$
    – The Photon
    Commented Dec 12, 2020 at 17:17
  • $\begingroup$ Great point the eyeball is not empty I didn't think about it actually. $\endgroup$
    – pnatk
    Commented Dec 12, 2020 at 17:35
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    $\begingroup$ It has everything to do with frequency. The term wavelength is just a convenient way of picturing a photon traveling at the speed of light with a certain frequency. $\endgroup$
    – Bill Alsept
    Commented Dec 12, 2020 at 17:40
  • $\begingroup$ @BillAlsept That is a misleading statement. $\endgroup$
    – my2cts
    Commented Dec 12, 2020 at 19:15
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    $\begingroup$ @my2cts How was that`misleading? A photon travels at the speed of light and it has a frequency. It’s not the photons speed that determines color. $\endgroup$
    – Bill Alsept
    Commented Dec 12, 2020 at 19:20

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Wavelengths are just easier to measure directly, so usually we find it more convenient to talk about wavelength rather than frequency.

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  • $\begingroup$ Do you have any hint how they are measured? $\endgroup$
    – pnatk
    Commented Dec 12, 2020 at 15:58
  • $\begingroup$ @pnatk, for example, reflect the light off of a diffraction grating (with a known spatial period) and measure the diffraction angle. $\endgroup$
    – The Photon
    Commented Dec 12, 2020 at 17:08
  • $\begingroup$ Yes that was my comment above. For example a green light has a wavelength of around 500nm but actually it has a frequency of over 600 trillion oscillations per second. Wavelength is convenient but really it’s all about frequency. $\endgroup$
    – Bill Alsept
    Commented Dec 12, 2020 at 19:55
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The wavelength $\lambda$, frequency $f$ and speed $v$ of any wave are related by the equation

$$v=f\lambda$$

The speed of light in vacuum is constant, so if the wavelength of light changes so does its frequency. It does not matter whether you associate the colour of visible light with its wavelength (in vacuum) or its frequency (in vacuum) - you get the same result either way.

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  • $\begingroup$ Speed of light in vacuum is constant. In a medium it's $n$ times lower, where $n$ is the refractive index of the medium. Thus, although the usual talks of wavelength of light implicitly assume the wavelength in a vacuum, this does appear confusing to novices. $\endgroup$
    – Ruslan
    Commented Dec 12, 2020 at 21:46
  • $\begingroup$ @Ruslan Thank you. I have edited my answer to clarify. $\endgroup$
    – gandalf61
    Commented Dec 12, 2020 at 21:52

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