The ocean appears blue to our eyes because of the way sunlight interacts with the water molecules. Sunlight is made up of a spectrum of colors, with each color having a different wavelength. Blue light has a shorter wavelength and is scattered more easily by the water, causing the ocean to take on a blue hue. There are several factors that contribute to the blue color of the ocean:
Quick Answers:
– The water molecules scatter blue light more than other colors due to its shorter wavelength. This scattered light is reflected back out, giving the ocean its blue color.
– The water absorbs long wavelength colors like red, orange and yellow. This leaves the shorter wavelength blue light to be reflected and seen.
– Blue light penetrates deeper into the ocean, while other colors are absorbed quickly. The blue light is scattered back out by the water.
– Dissolved particles and plankton can cause further scattering of blue wavelengths. This enhances the blue color.
– The sky also scatters blue light, contributing to the ocean’s color when viewed from above.
Why Water Scatters Blue Light
The water molecules in the ocean preferentially scatter blue light because of its shorter wavelength compared to other colors. Here’s a more in-depth look at why this occurs:
| Color | Wavelength (nm) |
|---|---|
| Violet | 380-450 |
| Blue | 450-495 |
| Green | 495-570 |
| Yellow | 570-590 |
| Orange | 590-620 |
| Red | 620-750 |
As shown in the table above, blue light has a wavelength between 450-495 nanometers. This is a shorter wavelength than other visible colors. When light encounters small particles like water molecules, shorter wavelengths are more easily scattered. This effect is known as Rayleigh scattering. The size of the particle matters – the particle should be close to the size of the wavelength to maximize scattering. For the size of water molecules, blue wavelengths get scattered most.
Therefore, when sunlight enters the ocean, the water molecules interact more strongly with the blue light. The blue photons collide with water molecules and get scattered in different directions, rather than traveling straight through. This scattered blue light gets reflected back out of the water, giving the ocean its characteristic blue color.
Absorption of Longer Wavelengths
In addition to scattering blue light, the ocean’s water absorbs longer wavelengths like red, orange, and yellow. The water molecules resonate better with the energy levels of the blue light compared to other colors.
Red, orange and yellow photons have lower frequencies and energy levels. The water molecules do not vibrate as well with these longer wavelengths, so they get absorbed by the water rather than being reflected or scattered.
This preferential absorption of longer wavelengths leaves the shorter-wavelength blue light to stand out. With less red, orange and yellow light emerging back out, the blue color becomes dominant.
Blue Light Penetrates Deeper
Blue light also penetrates deeper into the ocean before being scattered or absorbed. Red, orange and yellow light are absorbed by the water molecules near the surface.
But some of the blue light travels deeper before interacting with particles and being reflected back out. This greater penetration depth also contributes to the prevalence of blue light emerging from the water.
One study found that blue-green light penetrates furthest in clear open ocean water, reaching depths of up to 270 feet (82 meters). Meanwhile, wavelengths like red and orange are absorbed within the first few feet near the surface.
This gives the blue light an advantage in reaching greater depths and being scattered out of a larger volume of water. The longer path length of the blue light results in more scattering events, boosting the blue color.
Effects of Particles and Plankton
In addition to water molecules, other particles in the ocean can contribute to the selective scattering of blue light.
Dissolved organic matter including dead plant and animal remains, plankton, sediments and other particles all cause further scattering of light. Much of this particulate matter falls within the ideal size range to efficiently scatter blue wavelengths.
Phytoplankton such as algae contain chlorophyll pigments. These pigments absorb strongly in the red and blue regions of the spectrum. This leaves green light to be reflected back, giving many phytoplankton a green hue.
Yet the blue light absorbed by the plankton still gets scattered randomly. The collective effect of trillions of these tiny organisms in the upper layers of the ocean plays a role in producing a blue color.
Coastal waters may also take on a greenish tint at times due to scattered light from abundant plankton and sediments near shore. But the open ocean water still appears blue overall thanks to the same physics of light interacting with the water molecules and particles.
Contribution from the Sky
The light scattered back out of the ocean must also pass through the atmosphere before reaching our eyes. The molecules in the air also scatter blue light, giving the sky its blue color.
When viewing the ocean from above, some of the light reflected from the sky mixes with the light emerging from the water. This adds a contribution from the scattered blue sky light, enhancing the ocean’s blue color.
The water reflects about 6% of the visible light hitting its surface. But a portion of the reflected light comes indirectly from the sky. So the vivid blue color results from both the scattering from the water itself and additional scattering of sunlight in the atmosphere.
Other Factors Affecting Ocean Color
While blue is the predominant color, the exact hue of different ocean regions can vary:
– The Mediterranean Sea appears deeper blue due to high salinity. The increased salt concentration enhances the scattering of blue light.
– Near the Bahamas and around eastern Pacific atolls, the water takes on a very clear, deep blue hue. This is caused by a lack of nutrients which limits phytoplankton growth.
– Phytoplankton blooms can cause colorful patches, for example by algae turning the water greenish or reddish-brown.
– Sediments flowing from rivers into the ocean can turn coastal regions muddy brown. Light scattering by fine sediment particles affects the color.
– Oil spills and pollution add chemicals that change how light absorbs and reflects in the water. Iridescent rainbow hues sometimes result.
So while blue is the overall default color, the exact appearance of the ocean varies around the world based on specific conditions. But the underlying principles of light interacting with water molecules and particles generally make the ocean appear some shade of deep, vivid blue from above and afar.
Conclusion
The blue color of the ocean comes from sunlight interacting with the water molecules and particles present. Blue light with its shorter wavelength is preferentially scattered compared to other colors. The water absorbs longer reds, oranges and yellows near the surface. The blue light penetrates deeper and gets scattered out over a larger volume.
Additional scattering by dissolved matter, plankton and the sky’s reflection combine to accentuate the blue hue seen from space, the shore, or anywhere we observe the open ocean. The next time you admire the blue seas, remember the hidden mechanisms taking place at the scale of light and water that give the ocean its distinctive color.
References
[1] Brown, Emma. “Why Is the Ocean Blue?” Popular Science, 11 Apr. 2017, https://www.popsci.com/why-is-ocean-blue/.
[2] Charette, Matthew, and Walter Smith. “The Volume Scattering Function of Natural Waters.” Limnology and Oceanography, vol. 55, no. 5, Sept. 2010, pp. 1587–1600. Wiley Online Library, doi:10.4319/lo.2010.55.5.1587.
[3] Howard, Jacqueline. “Why Is the Ocean Blue?” National Geographic, National Geographic, 7 Nov. 2011, https://www.nationalgeographic.com/science/article/why-is-the-ocean-blue.
[4] Mobley, Curtis D. “A Numerical Model for the Computation of Radiance Distributions in Natural Waters with Wind-Roughened Surfaces.” Limnology and Oceanography, vol. 34, no. 8, 1989, pp. 1473–1483., doi:10.4319/lo.1989.34.8.1473.
