If Water Is Colorless, Then Why Does the Ocean Appear Blue?

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The blue color of ocean water is an intrinsic property and is caused by the selective absorption and scattering of white light. Therefore, when sunlight shines on the sea, all the colors of the rainbow go into the water.

Just like Jackie said, your science teacher has probably told you, “Water is colorless”, and your reaction is something like, “What’s wrong with our teacher? Is she colorblind or something?”

Is seawater blue or colorless?

Well, as impossible as this may sound, your science teacher is right. When you look at rainwater, tap water, or the water you drink, you will find that it is colorless. If that’s true, then why does seawater look blue if other types of water have no color whatsoever?

One explanation that people often give is that the color is a result of the sky’s reflection, but is that really true?

If seawater owed its color to the sky, it would actually have a much different shade of blue. Not only that, but it would look white on really cloudy days.

The Real Explanation behind the ocean’s blue color

While relatively small quantities of water appear to be colorless, the slight tint of water becomes a deeper blue as the thickness of the observed sample increases. The blue hue of water is an intrinsic property and is caused by the selective absorption and scattering of white light.

Therefore, when sunlight shines on the sea, all the colors of the rainbow go into the water. Red, Yellow, Green and Blue all flood into the sea.

However, the sea absorbs the red, yellow and green light, leaving only the blue light behind to reflect back out. Some of the blue light scatters off of the water molecules and that scattered blue light comes back out of the sea. This is the blue color of sea that we perceive, leading most people to believe that water is blue.

Maybe for your next field trip, you should take your science teacher to the beach and explain this cool phenomenon about seawater. If she still argues that the seawater does not look blue, give her a Rubix cube and ask her to solve it. That way, you’ll know whether she is colorblind or not!



  1. UCSB Scienceline
  2. University of Delaware
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About the Author:

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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