I thought the sea was blue because it reflects light from the sky but my chemistry teacher told me it’s because it absorbs red light. Which of these is correct? 
We had a thread on this last August. As I remarked then, the reason usually given is multiple scattering. Here’s an article on the blueness of water than supports the “red gets absorbed” argument:
if you have a white bathtub, when you fill it the water should have a pale green-blue tint.
Note that the scattering has a strong dependance on wavelength (to the FOURTH power!!!). such that shorter wavelengths like blue are scattered MUCH MUCH MUCH more strongly then red; hence the blue color.
So I should clarify, as I understand it, redish light doesn’t really get absorbed like mentioned above, it just doesn’t scatter as well and penetrates through the water.
The sea is blue because water is blue.
Isn’t the relatively uniform sky-blue color of the oceans as seen from space due to the reflecting-the-sky thing, though?
No.
Rayleigh scattering. The reflection of the sky in the ocean idea is incorrect; both sky and water are blue because blue(r) light scatters so much better then red(er) light when interacting with small particles like molecules
The sky is blue because of Rayleigh scattering. It doesn’t hold for water (Rayleigh scattering assumes widekly separated noninteracting scattering centers, and single scattering). Water doesn’t work that way. And it’s a different shade of blue.
I’m not familiar with the assumption that requires widely separated non-interacting centers. I’m mostly familiar with Raman spectroscopy that works very hard to eliminate the Rayleigh scattered photons from the spectrum. As I understand it even solids and liquids will have a Rayleigh scattered component of light.
I’m totally willing to accept your premise though, and after I’ve poked around for 2 min on the internet, it doesn’t look like Rayleigh scattering is the single best explanation for water being blue.
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And this, right here, is an example of why I like the DOPE.
Let’s see, for an old disabled fart at this time of day, my choices are Live with Dips***, Doctor Oz, Judge Judy, or the DOPE.
Thank you.
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Here is a page with the relevant assumptions:
http://www.lsinstruments.ch/technology/static_light_scattering_sls/rayleigh-gans-debye_scattering/
Basically, you have to assume that your input light isn’t significantly affected by the scattering, and that most interactions are single scatters. That holds true when you have dispersed scatterers(even in liquids or solids, if the scattering centers are separated within the medium), but not if they’re densely packed.