Why is the sea blue?
The sea looks blue mainly because water absorbs red, orange and yellow light and reflects or scatters blue light. Reflections of the blue sky add to the effect.
Sunlight contains all the colors of the rainbow. When it enters the ocean, water molecules absorb the longer red wavelengths first, within the top few meters. Green and blue light travel deeper. Some of that blue light is scattered back toward the surface and reaches our eyes, so deep, clear water looks blue.
On a sunny day, the surface also reflects the blue sky, making the sea look even bluer. Under a gray, cloudy sky, the same water looks gray or dull green. That is why the color of the sea can change from hour to hour with the weather and the angle of the sun.
Why some seas are green or brown
Tiny floating plants called phytoplankton contain green chlorophyll, which gives nutrient-rich coastal water a green tint. Rivers carry mud and sand that turn water brown near their mouths. Shallow water over white sand looks turquoise because light reflects off the bright seabed. Algae blooms can even make water look red.
Did You Know?
A glass of water looks clear because the amount of water is too small to absorb noticeable red light. You need several meters of water to see its natural blue tint.