I recall this question arising one day in my 12th grade Physics class (many moons ago). One student proposed the sky was blue since it reflected the sky. Many laughed since the teacher’s explanation seemed more scientific - that the water simply reflected the shorter wavelengths. Yet, how do we know which is correct? When the sky is gray, major bodies of water appear gray. Is this not a strong argument that the ocean reflects the sky? The title of this Scientific American article suggests it will clarify whether the ocean simply reflects the sky, but it fails to explain beyond a shadow of a doubt whether or not the ocean is reflecting the sky. How do we know for sure?
Clear, clean water preferentially absorbs wavelengths near red end of the visible spectrum, and transmits wavelengths near the blue end of the spectrum. Hit it with a whole bunch of different wavelengths, and only the bluish ones survive to be either received by the eye of a submerged observer, or reflected by a submerged object back up to the eye of an observer above the water’s surface.
You can eliminate the “the ocean reflects the sky” theory a couple of different ways:
#1: note that very shallow bodies of water (e.g. a glass of water) do not appear particularly blue. This is because the light is not passing through much water, so not much of the red wavelengths are absorbed.
#2: by placing the observer and the light source deep below the ocean’s surface, away from natural light. Assuming a broadband light source (i.e. emitting more or less equally at all visible wavelengths), the farther the observer is from the source, the more blue the source will appear (because more of the red wavelengths are absorbed during the longer trip through the water).
Its blue simply because water is very faintly blue. We may think that water is clear, but in reality it is very slightly blue. The more water you look through, the more blue it appears. This website Colors from vibrations | Causes of Color shows an image of looking through a column of water and noting the blue tinge.
Its as normal as saying greenish glass is green because that is just the color of that material.
A more scientific and detailed explanation is that the water molecules very slightly selectively absorb light from the other wavelengths and less so the blue light. Therefore the blue colored light is what gets through to your eye.
Machine Elf and Sigene have it right. A common misconception, often told by SCUBA divers, is that the light gets bluer the deeper you go, but it’s actually the further the light travels. You get the same effect looking from one end of a swimming pool to the other at a depth of one or two feet.
Water is blue … here’s a photo of ice that has had all the air bubbles pressed out of it … “Glacier from Somewhere” … if we have a large and deep enough lake, and the water is very very pure, this particular shade of blue really stands out, and it is a distinctive shade of blue IMEIO … Crater Lake in Oregon is an excellent example …
Yes, water itself is slightly blue, and in extreme conditions, that’s relevant. For most purposes, though, like a person standing on the shore and looking out at the water, water will appear to be the same color as the sky, whether that color be blue or gray.
I imagine for SCUBA divers, the Purkinje effect (everything looks blue in the dark) also is a factor.
But yes, water is a blue chemical.
It may be the oxygen molecules providing the blue color. At our Rocket Engine Test Facility the liquid oxygen lines ran in insulated troughs filled with liquid nitrogen to super-cool it. You could tell if there was an O2 leak because the LN2 in that trough would appear a beautiful sky blue.
Dennis
Have you ever seen an indoor swimming pool? The water is blue or blueish green. No sky to reflect, and the ceiling often isn’t blue either. That’s how we can tell, water is blue without reflecting anything.
Although liquid oxygen is blue, considering the concentration of dissolved oxygen in the ocean (say 6mg/L) that blue will be diluted by over 190,000 times and it’s color wouldn’t be perceptible to our eyes.
Some contaminants like certain types of finely crushed rock can get suspended in water and reflect certain wavelengths of light and make the water appear intensely blue or greenish.
Or maybe because they all have blue tiles on the bottom.
It’s all well and good to say that air is blue and that’s why the sky is blue… until the kid then asks why sunsets are red, and you answer that it’s because air is red. Which is exactly as true as saying that it’s blue. The simple fact is that the color of the sky is due to a very different mechanism than the color of most objects we’re familiar with.
The sky is blue. That is because the atmosphere scatters the light from the sun, and blue (having the shortest wavelength), is the only one that reaches the earth. Then the ocean reflects the sky.
Why am I the first person to mention this? I am not even the smartest person on these boards.
(Boy, water is slightly blue. I never even knew that. Probably not relevant to this question though. I know hydrogen peroxide is slightly blue. And it’s formula is H[sub]2[/sub]O[sub]2[/sub]. Not too relevant to the question. But I felt compelled to add it for some reason:).)
Under a raft, the water is a pale green, from the plant life I assume. The water appears blue from the top of the raft on a sunny day.
I’ve never noticed blue water to be blue.