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I know that different colors have different wavelengths, with the color blue having a shorter wavelength than other colors. My question is: When light is reflected off the surface of the ocean, does this shorten or lengthen the wavelength of the colors in the light spectrum? Thank you so much for your time!
Question Date: 2015-04-12
Answer 1:

It turns out that when light is reflected off of a stationary object, its wavelength doesn't change (if light is reflected off of a moving object, its wavelength can become shorter or longer, depending on how the object is moving; this effect is called blueshifting or redshifting).

The surface of the ocean is not moving (waves don't count - for redshifting or blueshifting to occur, the reflecting object must be moving VERY fast), and so the wavelength of the light it reflects doesn't change (in fact, if you look at the surface of a very still pond, it can behave remarkably like a mirror!). So you might wonder: what makes the ocean blue? Well, most of the time, the light we see from the ocean is not just light reflected from the surface, but light that has penetrated into the water and been scattered back out (this scattering happens as the light bounces off of water molecules or small particles in the water).

Now, this scattering also doesn't change the wavelength of light, but it does affect different wavelengths differently; in particular, shorter wavelengths scatter much more than longer wavelengths. Since the light that comes from the sun contains a range of wavelengths, this scattering process will tend to cause the shorter wavelengths (like blue) to be scattered back out of the water, while the longer wavelengths (like red) will tend to penetrate deeper into the water and get absorbed. So the blue color of ocean water comes from the preferential scattering of blue light rather than red, and not from any shifting of wavelengths. (This is also the reason the sky looks blue, by the way).

(You might have an objection: purple light has an even shorter wavelength than blue light, so why doesn't the ocean look purple? The answer is a complicated combination of the relative amount of blue vs. purple light that actually makes it through the atmosphere to the surface of the Earth as well as how our eyes perceive color.)

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