I was wondering if whether we were extremely lucky to have found spectral (absorption) lines of astronomical objects because they fell within the visible light range or if there something intrinsic about it. In the same direction, is there spectroscopy analysis within other light frequency ranges?
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2$\begingroup$ There are ultraviolet and infrared lasers. $\endgroup$– Mike DunlaveyCommented Sep 15, 2014 at 1:58
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2$\begingroup$ The more important question is whether we were lucky that some lines (visible or not) fell in a transmission window of the atmosphere - and the answer is basically "no" - spectral lines cover a very wide range of wavelengths. $\endgroup$– FlorisCommented Sep 15, 2014 at 1:59
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2 Answers
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The Balmer series for hydrogen is an example with some visible lines, and some lines outside the visible spectrum.
On the other hand, the Lyman series for hydrogen is completely outside the visible spectrum.
Since every element has an infinite number of spectral lines, it would instead be very unlucky if they all somehow fell outside the visible spectrum.
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There is no luck involved. Quite the opposite. Firstly, as many of the comments say, spectal lines arise in a very wide range of frequencies well outside the visible range. Secondly, and IMO more importantly for this answer, evolution is NOT random. And we see in the visible range because:
"Most" of the Sun's radiant energy is in this band, in the sense that if you look at the blackbody radiation curves for different temperatures (see below, which I swiped from the Wikipedia "Wien Displacement Law" page), the one for 6000K (the Sun's surface temperature) has its peak smack in the middle of the visible range. So there is naturally an evolutionary driver for an evolving organism's EM sensory system to sense where the sensed quantity (EM radiation in this case) is strongest.
Optical frequencies are one of the regions where matter interacts strongly with light. This is exactly the frequency range, for example, where metals begin to behave like lossy dielectrics rather than perfect conductors and where much of the spectrums of organic molecules lie. Therefore, as the light sensing system's evolution goes forward and becomes more elaborate, it will become most sensitive to the spectrums (the "colour" of the light transmitted and reflected by) of the molecules making up predators and prey of the evolving organism. Once this process began, it became ever more complex: organisms began using the spectrums of these organic molecules for sexual signalling (birds, butterflies, fish with four-colour, polarisation sensitive sight), angiosperms took advantage of this and evolved colourful flowers to dupe other animals into pollinating them and so on and so forth. We primates, for example, became one of the few mammals to rehabilitate our red receptors as we became diurnal and omnivorous: red helps detect and analyse fruits and berries.
So, in short, because there is a great deal of Sun's radiance as well as organic molecule spectal activity in the visible range, life evolved with evolutionary drivers grounded on these facts, and not the other way around.
answered Sep 15, 2014 at 2:49
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1$\begingroup$ Note that if you find the sun's irradiance as a function of frequency or energy rather than wavelength (that is, $dP/d\nu$ rather than $dP/d\lambda$, as you've shown) then the peak and most of the energy fall in the infrared. $\endgroup$– rob ♦Commented Sep 15, 2014 at 2:57
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$\begingroup$ @rob Still, its not far off: both say there is still plenty of intensity in the visible band, and, as I said, there is more than one evolutionary driver. Both the $\lambda$ and $\nu$ graph would show that light sensing has to evolve somewhere near the visible: it can't evolve in Xray or higher regions and is less likely to evolve in very far IR. It would be interesting to graph the spectra of all the amino acids and life-important molecules and see where they lie relative to the Sun's BB curves: I'm sure some biologist would have done this. $\endgroup$ Commented Sep 15, 2014 at 3:34
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1$\begingroup$ There's an important chemical-biological reason for our spectral vision range as well: there aren't any chemicals in the general organic collection with sensitivities in the mid-infrared or longer wavelength range (very small photon energies). It's not a matter of how much solar energy but rather the photon energy, as detection is a quantum effect. $\endgroup$ Commented Sep 15, 2014 at 11:45
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$\begingroup$ Hi! Thank you for your answer. It doesn't quite address my point - maybe my fault for not being able to express myself clearly enough. At any point I meant to bring evolutionary science into the discussion, nor to express that evolution is random. I am quite versed in the topic and thus I am not looking for any clarification in that respect. $\endgroup$ Commented Sep 15, 2014 at 16:04