As the radiation peak of the sun is in the UV region and since at around room temperature materials emit radiation at IR, I wonder why our eyes are not capable of using these wavelengths. I guess there is a reason why we exactly only see the region between these intensity peaks?


  • $\begingroup$ some quick googling showed that there are lots of easy to understand answers around (ceenta.com/news-blog/…), (scienceline.ucsb.edu/getkey.php?key=3019) $\endgroup$ – user438383 Aug 14 '20 at 9:07
  • $\begingroup$ The first link isn't revealing any information, just physical basics about light. The last two answers from the second link are, in return, quite interesting. Particularly, I wonder why visible light offers the possibility to notice (sharp) boundaries.. $\endgroup$ – Ben Aug 14 '20 at 10:04

Usually we don't ask why in biology because the explanation is always the same, it was good enough for survival. But here are a couple of explanations. The radiation peak from the sun is in the visible range of the spectrum, between 400nm-700nm with the highest point around 550nm as can be seen here or calculated from Wien's law and the sun temperature. That's why photosynthetic pigments use the visible spectrum and following them the rest of the ecological system.

Our retina blocks most of the UV light and water absorbs the lower part of the IR, illustrated very nicely in this article, figure 1.

It is true that materials on earth emit more radiation in the IR than in the UV-visible spectrum due to their temperature but:

  1. most objects have the same temperature approximately
  2. the soil emits a lot of radiation
  3. the radiation from the sun in the IR is high compared to the radiation of earth. This is the best graph I found, emphasizing the great difference between sun & earth intensities in the IR range there isn't a lot of variance in the wavelength intensity and therefore it is less useful. Notice how uniform the IR reflection is from this leaf spectral signature

To summarize:

  1. UV light is dangerous so we filter it out and not use it
  2. IR light is uniform and not useful
  3. Visible light is the peak of sun emission and therefore the most efficient range to use
  • $\begingroup$ Crystal clear answer. Thanks a lot! $\endgroup$ – Ben Aug 14 '20 at 11:21
  • $\begingroup$ The first sentence is good, the following explanations not so good. It doesn't explain why many birds & reptiles are tetrachromats and see in the ultraviolet (and some insects in the infrared), or why most mammals - primates being the exception) are dichromats, and don't see much of the red portion of visible light. $\endgroup$ – jamesqf Aug 14 '20 at 17:39
  • $\begingroup$ @jamesqf the question was about our, human vision. The topics you mention are interesting but out of scope in my opinion. I'd answer them in depth in a separate question if you post it. $\endgroup$ – Hachiloni Aug 15 '20 at 19:18
  • $\begingroup$ @Hachiloni: Human vision is not qualitatively different from that of other animals. The fact that many other animals DO see in IR and UV ranges is sufficient to demonstrate that your answer is not correct. Also note that photosynthesis does not use all of the visible spectrum, otherwise leaves would be black, not green. $\endgroup$ – jamesqf Aug 16 '20 at 17:15
  • $\begingroup$ Also re your point #2, IR light is most certainly not uniform. For instance, it's fairly easy to take infrared photographs with a modified digital camera. (You remove the built-in IR blocking filter, and install a visible light blocking one.) A search for "infrared photography" will give numerous examples, e.g. petapixel.com/2016/10/27/… $\endgroup$ – jamesqf Aug 16 '20 at 17:21

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