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I realize that the spectrum is man-made, and "visible" is a human-relative term. However, the scale is based on objectives, such as frequency and wavelength, and "visible" light is objectively unique in that it wholly absorbed by solid/opaque objects. EM waves on either side of it can pass through intervening media (albeit to different degrees). Is there a specific reason why the "colored" wavelengths are uniquely blocked by objects?

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    $\begingroup$ ""visible" light is objectively unique in that it wholly absorbed by solid/opaque objects. " is not true in that other parts of the electromagnetic spectrum are wholly absorbed by solid objects. Try getting UV through a brick wall. $\endgroup$
    – Farcher
    Apr 20, 2017 at 10:43
  • $\begingroup$ Isn't UV considered the very upper limit of the Visible section (just like IR is the lower limit)? Even if we "can't see them" it seems like that specific region is unique (maybe I'm wrong). If you go either way from there, very high energy or high wave lengths, it seems that objects become less of an issue $\endgroup$ Apr 20, 2017 at 10:54
  • $\begingroup$ Try tv/radio waves through salt water. $\endgroup$
    – Farcher
    Apr 20, 2017 at 10:56
  • $\begingroup$ I would say that we define "opaque objects" as those which block visible light. But there are also objects that are e.g. "opaque in the UV". $\endgroup$
    – Noiralef
    Apr 20, 2017 at 11:01
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    $\begingroup$ Just for the record, I find the terms "beginning" and "end" of the spectrum to be somewhat confusing. You can have frequencies ranging from incredibly low to incredibly high, we just choose a "spectrum" that shows frequencies we are familiar with and experience (there are likely physical limits due to some constraints at very small/large scales, but for the most part you can at least mathematically represent any frequency) a frequency spectrum has no beginning or end beyond where you choose to be cutoff points. $\endgroup$
    – JMac
    Apr 20, 2017 at 11:25

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I feel like Captain Obvious, but you seem to have it backwards.

Different objects (continuous media, conducting grids, whatnot) have transmission and absorbtion spectra, that is, they transmit various parts of the electromagnetic spectrum to different degrees. Our eyes, on the other hand, detect only a part of the radiation -- that part is what we call visible light, and the objects that block that light are (commonly) called opaque. Of course, opaque objects (for visible light) may be transparent to other frequencies (e.g. radio waves through walls), and transparent objects may be opaque to other radiation (e.g. UV through glass).

This is different from being solid: glass and bricks are solid, water and black ink are liquid, yet one of each transmits visible light and one doesn't.

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