system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is water blue in a large area but clear if you were to cup it up in your hands?

I'm sure it's because of refraction, but why does it refract blue in a large area?
gpt
Large amounts of water are blue for several reasons. 

Water is blue because it absorbs red light due to its chemical structure. When a photon with the amount of energy that makes it red hits a water molecule, it is absorbed and allows the water molecule to vibrate with its O-H bonds at a more excited state. 

This results in blue color overall. It's simply not visible in small quantities because the light hasn't passed through enough water for your eye to be able to detect the blue tone. If you took a gallon of water and stretched it into a long tube and looked through the end, you could see the blue. 

There are other factors when considering large amounts of water in nature, like reflection from the sky and scattering of blue light by suspended particles (eg, runoff from glaciers containing fine silt)