Why is the Ocean Blue?

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Have you ever wondered why the oceans are blue? In this lesson we'll explore this concept and what this has to do with wavelengths of light in the electromagnetic spectrum.

Light and Color

Light is pretty amazing stuff. It travels as both waves and particles, moves incredibly fast, and comes in lots of different forms. But my favorite part about light is color. The things we see don't actually 'have' color. And you've probably noticed this without even realizing it before. Think about it: does a rose look red in the dark? Does your blue shirt look the same under every light bulb? Is the sky red after the sun has finished setting for the day? Light plays tricks on us in this way, making us think that things have color and are that color all the time.

The electromagnetic spectrum describes the different wavelengths of light.
electromagnetic spectrum

The electromagnetic spectrum is a range that describes the various wavelengths and forms of light, and the part of the spectrum we see is called visible light. We see things in color, but the colors we see actually depend on the frequencies of light we can see and how that light is absorbed or reflected by objects. A rose looks red because it reflects red light, and its leaves look green because they absorb other wavelengths of light such as red and blue.

Don't believe me? Well, that rose that you see as red is not red to people who are color blind, as well as to most other organisms. For example, to a bee, which sees ultraviolet light instead of the visible light our eyes see, the rose looks much more like the green leaves surrounding the rose. This goes to show that color is a perception, not a cold hard truth!

The Blue Planet

But for us we do see things in certain colors, and one stunning example of this is the bright blue oceans that cover our planet. Earth is called the 'blue planet' because it is covered with so much blue water. But why do those oceans look blue to us? This again is a trick of light and our eyes.

Earth is called the blue planet because it is covered with water.
earth from space

When light hits the ocean water, the wavelengths of light at the red end of the spectrum are absorbed by the water as are other wavelengths such as violet. This leaves behind blue wavelengths to be reflected back to our eyes, making the ocean water look blue. Some other colors may also appear in ocean waters such as green-ish hues.

Ocean waters look blue because these are the wavelengths that are reflected back to our eyes.
blue ocean water

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