The sky’s beautiful blue color has fascinated humans for millennia. Ancient mythologies explained the sky’s hue as reflecting the moods of gods and goddesses. Later philosophers and scientists sought physical explanations for the optical phenomenon that makes the sky look blue.
As we now know, the blue color of the sky during daytime is due to Rayleigh scattering. As sunlight enters the atmosphere, nitrogen and oxygen molecules scatter blue wavelengths more than red and green wavelengths. The scattered blue light spreads across the sky, making it look blue from the ground no matter which direction you look.
Interestingly, while Rayleigh scattering explains the blue sky over land, different factors lead to the deep blue color of the open ocean. The ocean’s color depends on the water molecules and any particles or organisms suspended in the water. Pure water molecules alone absorb almost all sunlight except blue, leading to relatively little reflection and a very deep blue hue.
In this article, we will explore the science behind both the sky’s and the ocean’s captivating blue color. Understanding the light physics that paints the heavens and seas blue can make us appreciate anew the beauty of our planet.
Why Is the Sky Blue?
The sky’s blue color results from sunlight interacting with molecules in Earth’s atmosphere. As white sunlight passes through the air, small atmospheric particles scatter more blue wavelengths than red and green. This is called Rayleigh scattering.
What is Rayleigh Scattering?
Rayleigh scattering describes the elastic scattering of light by particles much smaller than the light’s wavelength. It was first modeled by Lord Rayleigh in the 19th century. Rayleigh scattering is highly dependent on wavelength, with shorter wavelengths scattering much more than longer wavelengths. This process strongly scatters violet and blue light coming from the sun, filling the sky with the scattered blue colors.
In Rayleigh scattering, scattered light retains its original wavelength. This differs from Raman scattering, where interactions between light and matter cause the scattered photons’ wavelengths to change.
Rayleigh scattering applies when particles are far smaller than light’s wavelengths – on the order of 1/10 the wavelength or less. In the atmosphere, major Rayleigh scatterers include individual air molecules – both nitrogen (N2) and oxygen (O2). These are much smaller than visible light wavelengths.
Larger atmospheric particles like dust and water droplets show much less Rayleigh scattering. Instead they reflect and refract light without selective scattering of blue. This explains why skies are less blue and more whitish on hazy days.
Why Do Shorter Wavelengths Scatter More?
Rayleigh scattering’s strong wavelength dependence arises from how scattering efficiency scales with particle size vs light wavelength. As derived by Lord Rayleigh, the intensity of scattered light is proportional to:
1/wavelength4
This 1/λ4 relationship makes shorter blue wavelengths scatter much more readily than longer red/orange wavelengths from the sun. Violet light scatters even more than blue. But our eyes are less sensitive to violet, so we perceive the sky as blue.
Rayleigh Scattering and the Colorful Sky
Rayleigh scattering not only makes the sky blue but produces other colorful sky phenomena:
- Red sunrises/sunsets – At dawn or dusk, sunlight travels longer through air, so more blue/violet light is scattered away, leaving mostly long wavelength red.
- White daylight sky – With the sun overhead, all colors scatter, mixing to produce a white daylight sky.
- Scattering angle – Light scattered at angles near 90 degrees from the sun’s rays appears blue. At smaller angles near sunrise/sunset, more red light remains.
In summary, Rayleigh scattering gives Earth both its blue skies and multi-colored sunrises and sunsets!
Why is the Ocean Blue?
While Rayleigh scattering produces the sky’s blue color, the primary factor making the open ocean blue is water selectively absorbing colors. Pure water molecules alone absorb almost all sunlight except blue, leading to relatively little reflection and a very deep blue hue.
Pure Water is Intrinsically Blue
Water’s intrinsic blue color results from its molecular structure. The water molecule’s absorption spectrum is such that red, orange, yellow, and green wavelengths are strongly absorbed. Blue and violet light are absorbed much less.
Therefore when sunlight enters pure water, most warm colors are quickly absorbed, leaving blue to be scattered back. The relatively low absorption in the blue part of the spectrum gives water its inherent blue color.
The molecular physics behind water’s light absorption properties have to do with its electron orbitals and hydrogen bonding. The specifics are complex, but the result is that water fundamentally absorbs long wavelength light more than short blue wavelengths.
Particles and Plankton Modify Ocean Color
Of course the ocean is not filled with pure water. Dissolved particles and marine organisms such as phytoplankton also influence its color through additional absorption and scattering.
In coastal regions, dissolved sediment acts like larger air particles during hazy skies. Sediment refracts and reflects light without blue selectivity, making coastal waters appear greener or browner. Phytoplankton also absorb more blue light through photosynthesis, reducing blue hues.
In the open ocean, water’s intrinsic blue absorption dominates since particles are sparse. This creates deep blue waters extending towards the horizon. Near islands, nutrients fertilize plankton growth, reducing blue tones.
Fun Fact: Polar Bear Fur is Optically Similar to Water
An interesting analogy for selective light absorption by water molecules is polar bear fur. The translucent hair strands scatter all colors except blue, making the bears appear white against the blue arctic sea!
Conclusion
The skies and seas both appear blue for very different reasons. Rayleigh scattering produces the sky’s blue color, while selective absorption by water molecules gives the open ocean its deep blue hues. A fascinating physics underlies the similar color of two quintessential images of nature. Next time you admire blue skies and seas, consider the light interacting with nitrogen, oxygen, and water to produce these signature colors of planet Earth!
