James Dreben 
#ELI5 Why is water see through?
I'd never thought about it like this. Wow.
UrbanEdm@Jdreben @Daveography Follow up question: why is water transparent at the same frequencies as air?
Greg Wellman@UrbanEdm @Jdreben @Daveography Even further follow up question. Why is the frequency band that both water and air are best transparent to also the band at which a very common class of star outputs most of its energy? (Hence making those wavelengths readily available to life evolving under such a star. Heck, one could continue with whether it's even possible for (a form of) photosynthesis to take place at significantly different frequencies.)
@gfwellman That seems less interesting. If air and water weren't the right amount of transparent at the right frequencies to match with our star, we wouldn't have evolved here to be able to ask the question. If it were otherwise, would life evolve in different gas and liquid mixes? Maybe.
I was more trying to get at the "what's the commonality between these two substances."
@UrbanEdm Hmm, I'm not sure there is an obvious commonality. Air is almost entirely made of non-polar molecules, while water has dipole. That makes a big difference at microwave frequencies, but not to "visible".
What I was getting at is the likelihood of life. It's very nice that such a common class of star outputs energy that can be chemically captured. If that energy couldn't reach the surface, life might be very different and likely less common. Presumably there's a minimum energy photon below which something like photosynthesis just isn't possible, so bad news for deep red dwarfs, etc.
2xfo@UrbanEdm @gfwellman
I think Greg may be alluding that only certain frequencies are really useful to help you see in water, mainly the ones that make it through our atmosphere
I think Greg may be alluding that only certain frequencies are really useful to help you see in water, mainly the ones that make it through our atmosphere