Meteorology and Weather
Oceans and Seas

Why ocean seems to be blue when water is colorless?



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ocean color

The ocean's color has two causes: water is a blue chemical, but also the water's surface is very smooth and reflective.

If you gaze downward into deep water, reflections are minimized and you can see the water's true color. Water is a blue material, or in other words it absorbs the red end of the color spectrum and transmits the blue end. If water had no color, then lakes and oceans would be grey or black.

The color of water is not fixed. Shallow water appears turquoise, while deep water appears blue/black. This happens because water transmits green light better than red, and blue light better than green. Shallow water transmits some green light but more blue light, so it appears turquoise. Deep water only transmits a small amount of blue light, so it appears blue-black.

Water is also shiny. If you stand on the shore and look at the ocean in the distance, then all you will see is the reflection of the sky. During sunset, the distant ocean will appear red. This occurs because whenever light reflects from water at a low and "glancing" angle, nearly all of the light is reflected. The water behaves as a mirror, and reflected colors wipe out the water's own color. This reflection is not the color of the water. For example, if a shiny red automobile reflects the blue sky, then the car is still red, and only the reflections are blue. And this shows us another way to see the true color of water: just look at the ocean when the surface is rough and not shiny. When the ocean is rough and covered with waves, then the sky-reflections are mostly gone, and we can see the water's color.

See also: Causes of Color

Journal of Chem. Edu. 1993: Why is water blue?

LSB University: Water absorption spectrum