Why is the ocean blue if water is clear?
The short answer
The ocean appears blue because water absorbs red light and reflects blue light. In small quantities, water appears clear because most light passes through it without significant absorption.
The long answer
How can water be both clear and blue?
Sounds like the start to a riddle, but it's actually the start to a lesson in physics and chemistry.
Let's start with a refresher on the physics of light and color.
Visible light is made up of a spectrum of wavelengths, each associated with different colors. High-frequency, shorter waves produce colors like violet, indigo, and blue, while low-frequency, longer waves generate yellow, orange, and red light.
Source: ZME Science
When a photon (a fundamental particle of light) hits an object, a couple things can happen*:
Absorption: The object's molecules absorb the photon's light energy and convert it into thermal energy.
Reflection: The object's molecules reflect some of the photon's wavelengths. This is how we perceive color, by the way; we see the wavelengths of light that are reflected back to our eyes.
Transmission: The photon passes through the object, allowing light to come out the other side.
It's important to note that these interactions aren't mutually exclusive. Light can hit an object and do multiple things. For example, an object can absorb low and high-frequency wavelengths, reflecting mainly green light (this is why most plants are green).
Why is water clear?
Okay, with that quick lesson on light under our belts, let's tackle why water is clear.
We perceive water to be clear in small quantities because most of the light transmits through it.
Water (H₂O) is a molecule made up of two hydrogen atoms and one oxygen atom. The hydrogen atoms form strong bonds with the "parent" oxygen atom (the dark grey lines in the diagram below).
"Mundy2" by Thomas Brueckner is licensed under CC BY 2.0.
In liquid water, there are also weaker bonds between the water molecules themselves (the dotted grey lines in the diagram above). These looser bonds constantly form and break, creating tiny gaps between these molecules for an incredibly brief femtosecond (one-quadrillionth of a second!).
These gaps allow most light to pass between water molecules, making water appear transparent in small quantities.
Why is the ocean blue?
In small quantities, water appears clear because most light passes through. The key word being "most" — but not all!
When light does hit a water molecule, two things happen:
Water absorbs low-frequency red light, converting it into heat.
Water reflects high-frequency blue light.
In other words, water reflects blue light because it can't absorb those wavelengths, giving water its blue color.
We perceive the blueness of water best when there is a lot of it. There are three main reasons why more water means more blueness.
Reason #1: There is more opportunity for light to hit water molecules.
With more water, there are more chances for photons to hit water molecules instead of passing through. The more photons that hit and get absorbed by a water molecule, the more blue light is reflected back.
If it helps, here's an analogy: Imagine there's a chain link fence in front of you, and you throw a handful of marbles at it.
While some marbles would hit the fence and bounce back at you, most of them would pass through the chain link fence. This is why water is clear in small quantities — most of the marbles (photons) are passing right through.
But if you had 10 rows of fences, most of the marbles would bounce off one of the fences and not make it through. The more water there is, the more opportunity the photons have to hit a water molecule.
Reason #2: The scattering effect enhances the blue color.
Scattering is when light interacts with particles causing it to deviate from its original path. After water molecules absorb red light and reflect blue light, the blue light scatters in many directions, enhancing the blue color.
The different paths of the light in the water: reflection, scattering, absorption. Source: Citizens’ Observatory for Coast and Ocean Optical Monitoring
As light goes deeper into the water, there is more water to scatter the blue light, making deeper water appear bluer.
(Minor) Reason #3: There is a reflection of the blue sky.
It's a common belief that the ocean is blue because it reflects the sky. Although the reasons above play a much bigger role, the sky's reflection does impact water's color.
Check out this image below comparing an ocean under white clouds to a clear sky. The side without clouds appears more vibrantly blue.
"Ocean color 2007" by Ulmanor is licensed under CC BY-SA 3.0.
Again, this effect is less significant compared to absorption and scattering, but it still contributes to the ocean's blue color.
*Note: Light can also refract (i.e. bend) as it hits an object. But that opens up a whole other can of worms (ever wonder why a straw in a cup of water appears bent?), so I decided to leave it out. If you want me to answer, “Why does a straw look bent in a glass of water?” let me know!
Check out some other curious questions:
Sources
Aldridge, D. (2015, October 21). If water is clear, why is the ocean blue?. Words in mOcean. https://wordsinmocean.com/2015/10/21/if-water-is-clear-why-is-the-ocean-blue/
Citclops. (n.d.). What is water transparency?. Citclops. http://www.citclops.eu/transparency/what-is-water-transparency
Fava, M. (2022, June 7). Why is the Ocean “blue”? Ocean Literacy Portal. https://oceanliteracy.unesco.org/blue-ocean/
Henderson, T. (n.d.). Light Absorption, Reflection, and Transmission. The Physics Classroom. https://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission
Hijab, J. (2023, February 3). Water is clear and transparent for three simple reasons. Life’s Chemistry Press. https://www.lifeschemistrypress.com/category-water/water-is-clear-and-transparent-for-three-simple-reasons/
McVean, A. (2020, January 31). Why is the Sky Blue? Or Better Yet, Why is the Ocean Blue?. Office for Science and Society. https://www.mcgill.ca/oss/article/environment-general-science-you-asked/why-sky-blue-or-better-yet-why-ocean-blue
Mihai, A. (2013, April 2). The real color of water is blue — here’s why. ZME Science. https://www.zmescience.com/feature-post/natural-sciences/physics-articles/matter-and-energy/the-real-color-of-water-blue/
Noy, G. (2023, October 15). What is Light Scattering?. 3DOptix. https://www.3doptix.com/blog/what-is-light-scattering/
Woods Hole Oceanographic Institution. (2023, June 5). Why the ocean is blue?. Woods Hole Oceanographic Institution. https://www.whoi.edu/know-your-ocean/did-you-know/why-is-the-ocean-blue/