Light and the Color of Water

What is visible light?

The entire range of different kinds of light including the ones the human eye can see is called the electromagnetic spectrum.  What we can see is called visible light. A rainbow shows the colors of visible light. Visible light has wavelengths that range between 400 and 700 nanometers (one billionth of a meter).

Visible light in the ocean

Visible light penetrates into the ocean, but once past the sea surface, light is rapidly weakened by scattering and absorption.  The more particles that are in the water, the more the light is scattered. This means that light travels farther in clear water.  Absorption can be caused by phytoplankton which use the light for photosynthesis, particulate matter in the water, dissolved material in the water, or the water molecule itself.

Dropping M&Ms in the ocean

The light energy of some colors is absorbed nearer to the sea surface than the light energy of other colors.  The dimming light becomes bluer with depth because the red, yellow, and orange wavelengths have already been absorbed.  We typically see the ocean as having a blue-green color because these are the wavelengths that penetrate the deepest and are scattered back to our eye.

							Click on the M&Ms below to see how fast each color fades as the M&M falls through the water.
0m
1m							by 1 m about 60% of the light is absorbed.
10m							by 10 m about 85% of the light has been absorbed.
50m
100m							by 150 m about 99% of light has been absorbed.

The upper 100 -200 m of the ocean  is called the photic zone (photo = light).   Beyond this depth, light does not penetrate, and it is pitch dark. All the production of food by photosynthetic marine plants occurs in this thin surface layer.

Test Your Understanding

Why do tube worms living in water depths of 3000 m (9000 ft) look red in photographs such as the one shown to the right?

Check your answer below to see if you are correct.