Seawater… It’s a Blue Ocean


However, here is what is amazing… you take a glass and dip into the Gulf – raise it to observe – and it is clear… like a glass of water. There is no color to it at all. The emerald greens and cobalt blues are gone. What happen? We say the sea is blue, but what color is it really?

The emerald waters and white sands of the northern Gulf coast.
Photo: Molly O’Connor

Actually, it is clear – water has no color. So what makes the greens and blues we see? To answer this we need to understand a little about light. Let’s try this without getting too “sciency”. Light is energy moving in the form of a wave – radiating everywhere. Many are familiar with ocean waves and know that some have long wavelengths (distance from the crest of one wave to another) and these create the low swells. Others have shorter wavelengths creating choppy conditions. Light moves in the same way. The long wavelengths are the red, orange, and yellows. There are light waves longer than red but we cannot see them. The next longest is what we call infrared, which can be seen if using night vision goggles. The shorter waves are the green, blue, and violets. A little shorter than violet is ultraviolet, which some animals can see but we cannot. White light, sunlight, are all waves at once, what we can see and what we cannot. This is why it is so bright, a lot of energy at once.


As this white light from the sun meets the ocean, much of it is reflected back as it meets the more dense water. This creates the very bright glare we experience and the need for sunglasses. Light that is not reflected enters the denser world of water. Here the light waves begin to bend – or refract. This causes objects underwater to appear to be in a location where they are not. It also makes them appear larger and closer than they really are. Many of us have experience this change when are gigging for flounder or trying to catch crabs with a net. However, there is another change – absorption.


There are particles of salt, sediments, plankton, and others drifting in the water. These particles absorb some wavelengths of light and reflect others. In the ocean, the long waves of light (red, orange, and yellows) are absorbed first, reflecting greens and blues. You can see red and orange objects to about 10 feet where they become faded, and then to 30 feet where they are completely absorbed. The world turns green and blue, the emerald green and cobalt blue we were talking about. In underwater videos, the colors only return when artificial white light from the camera is used.

This diagram shows how the long waves lengths of light (red and yellows) are lost in shallow water.
Image: NOAA

Green light is longer than blue. Thus green disappears (is absorbed) at greater depths. This explains the “green” of the shallow water and the “blue” of the sea. Large objects, such as seaweed mats or coral heads, appear a rusty red-brown color to us from the surface – and single to the alert boat captain to avoid.


It is a blue ocean – a blue planet, but because of the amazing way light works. Our next topic… where does the salt in the sea come from?


Posted: June 30, 2017

Category: Coasts & Marine, Natural Resources
Tags: Coastal Habitats, Seawater

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