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The Sound of Science - "The Color of the Sea"

Welcome to The Sound of Science on WNIJ.

We’re answering listener questions this month and today we have another question on color. Neeha’s question is, “Why does the sea look blue?"

Well, Neeha, the short answer would be because water is blue. 

Wait a minute, my bottle of water is clear, not blue. What’s going on?

Water is a very faint blue so it appears clear in your bottle. The deeper the water, the deeper the blue.

But not all lakes and oceans are the same color. Why not?

That was the short answer. Sunlight is made of all the colors of the rainbow.  Water absorbs all colors, but red and green are absorbed long before blue, so unless you are deep enough for all light to be absorbed, water is blue. But rivers, lakes and oceans are not pure water.  Other particles in water reflect or scatter colors depending on their own interactions with light. Some mountain lakes, for example, are known for their brilliant turquois color caused by finely ground rock in the water reflecting and scattering other colors, shifting the lake towards green.  A shallow body of water can also look turquois when light reflects from a sandy bottom.   

Scientists can use satellite images to study the health of bodies of water based on color. Algae and sediment each contribute to the color of sea water. The chlorophyll in algae absorbs red and blue light and reflects green. Sea water with algae will appear blue-green to green depending on how much algae is in the water. Sediment absorbs and reflects many colors, making water brown.

I thought the sea was blue because it reflects the sky.

It can sure look that way, but water is blue, with or without the sky’s reflection. Cavers can see this clearly when illuminating an underground lake.

Keep your questions coming by emailing them to stemoutreach@niu.edu.

This is the Sound of Science on WNIJ, where you learn something new every day.

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