# Why is the atmosphere transparent in the visible spectrum?

One of the great 'coincidences' in physics is that the Sun happens to shine most brightly at exactly the wavelengths our eyes can see; it's an easy explanation that our eyes evolved to make the most out of what was available.

However, this spectral window is also one of the few at which our atmosphere is transparent, with Rayleigh scattering and the ozone layer protecting us from the UV and strong water vapour absorption in the IR until way into the radio range.

My question is: is this a coincidence? Were there geological factors that made it so? Or is it just lucky chance? (I also notice other planets in our neighbourhood weren't quite so lucky.) Or had atmospheric absorption been different our eyes would have tuned to whatever spectral window were brighter as a whole?

• Pretty sure it's the last option you chose: we see in the "visible" spectrum because there's plenty of it around! May 30, 2012 at 2:53
• May I suggest you consider a biophysics tag for this question May 30, 2012 at 4:14
• The atmosphere was created by living species that would adapt to the environment. species adapt to the environment but they also affect it. Kostya's answer shows that a lot of the absorption is done by water vapour (and oxygen), which was produced mostly from living things. And the atmosphere and sun were once part of the same nebula. Other planets weren't in the "goldilocks" zone, where water could reasonably be a gas, liquid and solid on the same planet. So the atmosphere needs life, which needs the atmosphere/water, which relates back to the intensity of the Sun May 30, 2012 at 12:24

Gases can only scatter light strongly if it matches a quantum transition (Rayleigh scattering doesn't involve quantum transitions but it's relatively weak). Quantum transitions can be rotational, vibrational or electronic (strictly speaking rotational and vibrational transitions are usually combined). Rotational/vibrational transitions have an energy that is generally in the IR range (which is why CO$_2$ scatters IR light) and electronic transitions have energies in the UV range (which is why ozone scatters UV). So there's a gap in the visible range. You say: