Question: Most optics texts will mention that alkali metals can become transparent in the near ultraviolet in the sections on reflections from metals, plasma frequency, and electron density. I remembered this again just now when I saw Table XXVII in Born and Wolf's Principles of Optics (6th ed.) and saw that the critical wavelength for cesium has an observed value of 4400Å, which is plain old blue light that we see all the time. The table caption says:

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> The critical wavelength $\lambda\_c$ below which the alkali metals become transparent, and above which they are opaque and highly reflective.
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...*alkali metals become transparent*... This sounds like it would be quite amazing to actually see - metal that becomes transparent in blue or even near-UV light! I would like to see this - even an image published somewhere - anything! Or if it is actually not really true as stated in Born & Wolf, what else is there to consider?

I think I've seen pieces of cesium under mineral oil, and plenty of pictures on the internet and it just looks like metal. Definitely not transparent for blue light in bulk from what I've seen.

Is the 4400Å value wrong, or am I misunderstanding something, or would the cesium still need to be relatively thin or perhaps very cold to be detectably transparent in blue visible light? Are there any examples of this surprising effect due to the low plasma frequency (large critical wavelength) that can be linked to or shown here? 

*Is there some data I can see that's not behind a paywall? A photo of blue light passing through bulk cesium metal?*

Answer: 
Simple metals are transparent at short wavelengths. It is maybe a problem that physicists do not usually write what their samples look like (chemists often note color and smell). I found this old [paper from 1931](https://www.jstor.org/stable/95606?seq=1#metadata_info_tab_contents) that starts: 

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> It is a matter of common knowledge that an ordinary gold leaf appears green by transmitted light while silver appears blue.
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I did not know this "common knowledge" about silver, such thin foils are not common. I have seen it for gold, but I cannot find photos. It would be very illustrative to have a photo of transparent films of the alkali metals but those are a bit difficult to prepare. I found a [movie about alkali metal dissolved in ammonia](https://www.youtube.com/watch?v=Hs-MDXWYiY0): first blue (solvated electrons are blue), then "bronze".

There should be some gold leaf around, I will look for it and try to make a photo.
It is absolutely accurate that two continuous functions (the magnitude and imaginary component of the optical index, in this case) will cross if they trend in opposite directions. Therefore, there must be a very specific wavelength at which the the real and imaginary components are exactly equal.

There are many reasons you may not see the transparency for practical reasons. Metal purity being one. Phonon scattering being another...