eli5 Why is water transparent in a glass but blue when it is in the ocean?


My wife gave birth to a philosopher. If you come up with an experiment I can show my 6-year-old son I’d appreciate it.

In: Earth Science


My understanding is that water in the ocean is blue/green because its reflecting the sky on a background of darkness. Keep in mind that the deeper you go in the ocean, the less light there is, so eventually you reach pitch black. So the surface layers of the water are effectively lying over that black level, and that forms a sort of mirror (very sort of) which results in a sort of reflection of sort of the colour of the sky.

There is a spectrum of colours in the light that goes by the acronym VIBGYOR. White light consists of seven colors namely Violet Indigo Blue Green Yellow Orange and Red.
Atmosphere consists of air that contains so many gases. When light (from the sun which is white) enters the atmosphere it collides with all kinds of particles. Due to this, colours with different wavelengths from the spectrum filter out except the colour blue that our eyes make out because of the distance and angle of the sun throughout the day. At dawn and dusk (when distance between sun and observer is greater than in the noon) you will see red tint because red colour has more wavelength than all the other colours.

For the glass, you can see it from the side, but the ocean you cannot. The surface of water in glass will also reflect the colour that is directly overhead it. Also, the size of glass is so small that you could easily change your perspective to take a look at it which may cause different colours from different POVs.

Simple answer. It’s not transparent. It’s 99.9% transparent with a .1% blue hue. Add up a bunch of 0.1% and you get a really blue color

There’s a very separate issue of the ocean’s surface reflecting the sky’s colors back to our eyes as we watch from the surface. On blue-sky days, the ocean surface looks bluer; on gray-sky days, grayer. The ocean surface boundary with the air is the cause of that, optically. For example, when the ocean dances with gold and red colors at sunset.

Independent of that, Liquid water absorbs slightly more of the other colors, compared to [absorbing not as much bluish light](https://en.wikipedia.org/wiki/File:Absorption_spectrum_of_liquid_water.png).

But still, the total amount of light absorbed is a very tiny amount! A small amount of water barely absorbs any, so it seems clear.

*Deeper water has more chances to absorb passing light. That’s why divers are surrounded by more bluish light as they dive deeper.*


Here’s a wonderful experiment for a kid, for a vivid look at light absorption in a bulk liquid, to imagine what a diver sees: use a small amount of food coloring or a colored artificial drink in two identical clear glass cups.

In one cup, put just enough liquid to spread out on the bottom. In the other cup, fill it up! Visually compare the vividness of the colors. The thin layer will look almost clear.

If the sun is shining, go outside and compare the colors cast upon a white surface on the “shadow” side. Recheck the emptier cup’s colors, each time you add a bit more liquid.

Water absorbs red and infrared (and to a lesser extent yellow and orange) better than other frequencies of light and is almost transparent to blue and cyan. White light minus the red looks blue or greenish blue. The more water light passes through, the more noticeable this gets; deeper water even filters out the green leaving only the deep blue. This is why clear seas, especially in the tropics, seem to go from green to turquoise to dark blue as you get farther from shore and more and more of the non-blue wavelengths are absorbed.