Happy new year seeker! That's a nice fundamental question to start the year.
Thomas Young is the person you're looking for, who established convincingly the wave character of light and determined the wavelength of visible light. With these results he also showed that different colors of light have different wavelengths.
Being picky, the wavelengths were not actually assigned but measured, which helped to make the wave theory convincing (eventually). This was done with an interferometric setup, where light is split and remixed after differing path lengths. In particular an etalon based on a convex glass lens touching a flat glass was used. In fact the already old measurements from Newton on such a setup were used for Young's interferometric analysis!
Much later, in 1960, the ability to determine wavelength was so advanced that the wavelength of light from a particular atomic transition was used as the definition of a meter. So at that point, one could also so that the wavelength was assigned. In the meantime since 1983, the meter is redefined on the basis of the speed of light.
The textbook "Optics" from Hecht and Zajac has a good and brief discussion of this history. And much more detail if needed must be available on the web if needed.
Thanks for the response, and Happy New Year to you too!
Ok, I'm following what you've said so far. I have a copy of Optiks (Newton), and saw the measurements that you speak of.
We have Fresnel, Abbe, and Young, etc. Working with ratios,
nλ/d = x/L
λ is the wavelength of the light d is the separation of the slits n is the order of peaks x is the fringe distance L is the distance from the slits to the screen
we can calculate the λ of the light.
So, when Fraunhofer worked out the spectral lines from the Sun (which is the first I find of specific #s, and elements), was this what he was doing? IE. did he separate some line, like Hα @ 656.281, and basically run it through a double slit, making it interfere, and measure the fringes?
Part of the reason for asking this is to estimate the accuracy of those measurements. How many decimal places, for example? (I've read there are some limitations, but don't know what they are)
Yes, Fraunhofer's measurements must be the first ones associated with spectral lines that allowed high accuracy. The numbers from Young's results are good but can only be associated with the perceived color. But those were apparently the first to even establish the order of magnitude. Fraunhofer used a grating, (an array of multiple periodic slits rather than just 2), which is required to get sufficient resolution to separate the lines. Otherwise they would be too broad and weak. The grating itself provides the separation or diffraction of the spectrum.