What is it about the ocean that makes it look blue when it reflects the sky?

Category: Chemistry
Published: December 20, 2012

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

The ocean is not blue just because it reflects the sky. The ocean is mostly blue because water itself is blue. In a Journal of Chemical Education paper titled "Why Is Water Blue?" by Charles L. Braun and Sergei N. Smirnov, water is shown to have a slight intrinsic blue color. It takes a large quantity of water, like in the ocean, for humans to notice the blueness. In household uses, like when drinking a glass of water, we use far too little water to notice its blueness. As a result, we think it is always clear. According to Braun and Smirnov, water absorbs red light due to vibrational transitions of the molecules, leaving the blue light to reflect back. For the same reason, large volumes of snow and ice also have a blue tinge. Sensitive laboratory equipment verifies the faint blue color of water even when only a cupful is present.

The ocean is blue because water is blue. Even when the sky is gray, the ocean is still blue. Public Domain Image, source: NOAA.

Note that if the ocean's surface is calm and you look at in from a low viewing angle, then some of the blue color on the water's surface is the reflection of the sky. But the reflection of the sky does not totally account for the blueness of water.

Topics: absorption, blue ocean, color, ocean, ocean color, ocean reflects sky, water, water is blue