The sky is blue. The ocean is, too. But the reasons why are very different.

If you’ve ever been curious about the world you live in, you’ve probably wondered why the sky is blue. It’s a question that most children ask at some point in their lives, but the answers that adults give them often don’t align with our physical reality. Some of the more common incorrect answers that people often give in response include:

• that sunlight has a blue tint,
• that oxygen itself is a blue-colored gas,
• or that the sky reflects the oceans, which are blue.

While none of those answers are correct, that last attempt brings up a related question that people often wonder about: why are the oceans blue?

As seen from space, planet Earth is often described as a pale blue dot, but it’s only the liquid bodies of water ⁠— dominated by Earth’s oceans ⁠— that appear blue-hued. The continents, clouds, and ice caps don’t appear blue at all. That allows us to conclude that it’s the oceans, rather than the atmosphere, that give our planet its overall blue complexion. For thousands of years, humanity had to simply accept these properties of our world as observed facts: we knew them to be true, but didn’t understand the physical cause behind them. With the advances of modern science, however, we now fully understand why both the skies and oceans are blue.

Contrary to what you might have read, there’s no one single factor responsible for Earth’s blue skies.

The skies aren’t blue because sunlight has a blue tint; our Sun emits light of many different wavelengths, and that light sums up to be a net white color.

Oxygen itself isn’t a blue-colored gas, but rather is transparent to light: relatively equally across all of the visible wavelengths.

However, our atmosphere is made up of much more than the nitrogen and oxygen that composes 99%, by mass, of the gaseous-phase matter that surrounds our world. There are a myriad of molecules and larger particles in our atmosphere that play a major role in terms of physics, scattering light of different wavelengths by different amounts. The ocean plays no role in the color of the skies, but the sensitivity of our eyes absolutely does matter: we do not see reality as it is, but rather as our senses perceive it and our brain interprets it.

These three factors:

• the Sun’s inherent light,
• the scattering effects of Earth’s atmosphere,
• and the response of the human eye,

are what combine to give the sky its blue appearance.

When we pass sunlight through a prism, we can observe how that light splits up into its individual components, with different colors (and different wavelengths) of light bending and refracting by differing amount. The highest-energy light that’s a part of sunlight happens to also be the shortest-wavelength (and highest-frequency) light, while the lower energy light has longer-wavelengths (and low-frequencies) than its high-energy counterparts. The reason light splits up at all is because wavelength is the critical property that determines how light interacts with matter.

The large holes in your microwave oven’s door allow short-wavelength visible light to freely pass through them, enabling you to see the food or drink inside, but keep longer-wavelength microwave light in, reflecting it, and enabling the food or drink to reheat and/or cook. The thin coatings on your sunglasses reflect ultraviolet, violet, and blue light, but allow the longer-wavelength greens, yellows, oranges, and reds to pass through: protecting your eyes while giving the lenses an apparent tint.

Similarly, the tiny, invisible particles that make up our atmosphere — molecules like nitrogen, oxygen, water, carbon dioxide, and argon atoms, plus any particulate matter — scatter light of all wavelengths, but preferentially are more efficient at scattering bluer, shorter-wavelength light.

There’s a physical reason behind this: all the molecules making up our atmosphere are smaller in size than the various wavelengths of light that the human eye can see. Whereas the light we see ranges between approximately 400 and 700 nanometers, the size of an oxygen or nitrogen molecule is more like 0.3 or 0.4 nanometers. The shorter wavelengths — wavelengths that are closer in size to the sizes of the molecules present — will scatter more efficiently than the longer, more distant-in-size wavelengths. That leads to the quantitative description we have for this scattered light today; the law it obeys is known as the law of Rayleigh scattering.

From the perspective of how efficiently light scatters, it turns out that the violet light at the short-wavelength limit of what we can see scatters over nine times more frequently than the red, long-wavelength light at the opposite spectral end of our vision. This is why, during sunrises, sunsets, and lunar eclipses, red light can still pass efficiently through the atmosphere, but the bluer wavelengths of light are practically non-existent, having been preferentially scattered away almost completely.

Since the bluer wavelengths of light are more easily scattered by the atmosphere, any incoming direct sunlight will become redder and redder the more atmosphere it passes through. As a result, depending on the time of day, the Sun can take on a yellow, orange, or even red appearance, as the bluer light coming from that direction gets efficiently scattered away. The remainder of the sky, however, will be illuminated by indirect sunlight: light that strikes the atmosphere and then gets redirected towards your eyes. The overwhelming majority of that light will be blue in wavelength, which is why the sky appears blue during the day.

The sky can only take on a redder hue if there’s enough atmosphere to scatter a sufficient amount of that blue light away before it reaches your eyes: a phenomenon most likely to occur where our line-of-sight is thickest through the atmosphere. If the Sun is below the horizon, all of the incoming sunlight light has to pass through large amounts of atmosphere. The bluer light gets scattered away, in all directions, while the redder light is far less likely to get scattered, meaning it takes a more direct path to your eyes. If you’re ever up in an airplane after sunset or before sunrise, you can get a spectacular view that demonstrates this effect.

This is enough to explain why sunsets, sunrises, and lunar eclipses are red, but might leave you wondering why the sky appears blue instead of violet. After all, the shortest wavelength light that we can see is violet, not blue, and yet the sky appears blue almost everywhere and violet nowhere during most times of the day.

It turns out that there actually is a greater amount of violet light coming from the atmosphere than blue light, but all of the colors are mixed in there, to some degree, as well. However, what we see depends on the response of the human eye, not just on the inherent properties of the light. In most humans, your eyes have three types of cones (for detecting color) in them, and along with the monochromatic rods, the signals from all four receptor types get interpreted by your brain when it comes to assigning visual colors and brightnesses.

Each type of cone, plus the rods, are sensitive to light of different wavelengths, but all of them get stimulated to some degree by the sky. Our eyes respond more strongly to blue, cyan, and green wavelengths of light than they do to violet wavelengths of light. Even though there’s more violet light than blue light, it isn’t enough to overcome the strong blue signal our brains deliver, and that’s why the sky appears blue to our eyes.

The explanation for the blue oceans, on the other hand, are an entirely different story. If you take a look at the planet as a whole, with a view such as the one you get from space, you’ll notice that the bodies of water we have aren’t a uniform blue, but rather vary in their shade based on the water’s depth. Deeper ocean waters appear to have a darker blue color; shallower waters close to the edges of continents or over freshwater lakes have a much lighter, paler blue color in appearance.

You’ll notice, if you look closely at a photo like the one below, that the watery regions bordering the continents (along the continental shelves) aren’t the same deep blue as the rest of the ocean, but rather make a more cyan shade of blue, with the color differences appearing particularly prominent around the perimeter of the Gulf of Mexico.

Some people simply don’t believe it. They insist that “the oceans are blue because they reflect the skies,” while others insist it’s the other way around, and “the skies are blue because they reflect the oceans.” We already established the physical reasons for the sky’s blue appearance, but we can go through a similar exercise for the oceans.

If you were looking for a more direct set of evidence that the oceans themselves appear blue, you could try diving down beneath the water’s surface and simply recording what you see. When we do this, taking a photograph underwater in natural light — i.e., sunlight, without any artificial light sources — we can immediately see that everything takes on a bluish hue.

The farther down we go, as we reach depths of 30 meters, 100 meters, 200 meters and more, the bluer everything appears. This makes a lot of sense when you remember that water, just like the atmosphere, is also made out of molecules of a finite size: smaller than the wavelengths of any light that we can see. But here, in the depths of the ocean, the physics of how light and molecules interact is a little different in its behavior.

The gaseous atmosphere is much lower in density than a liquid ocean is; our oceans are around 1000 times as dense as our atmosphere is at sea level. Whereas the large space, on average, between air molecules means that scattering occurs very easily, the molecules present in an aqueous environment instead are much more prone to absorbing light.

Water, like all molecules, has a specific, wavelength-dependent set of preferences for the types of wavelengths it will absorb. Rather than having a straightforward wavelength dependence like our atmosphere does for scattering (more efficiently scattering short wavelengths and less efficiently scattering longer wavelengths), water can most easily absorb infrared light, ultraviolet light, and red visible light.

This means if you head down to even a modest depth, you won’t experience much warming from the Sun. You’ll largely be protected from UV radiation, and things will start to gain a blue tint to them, as the red light is taken away. As you head down to even deeper depths, the oranges go away, too.

At still deeper depths, the remaining wavelengths of light start to get cumulatively absorbed as well. The yellows disappear next, followed by the greens and violets. Only after we head down to depths of multiple kilometers does the blue light truly disappear as well; it’s the last visible light wavelength to do so.

This is why the deepest ocean depths appear a deep, dark blue: because all the other wavelengths have been more completely absorbed away. The deepest blues, unique among all the wavelengths of light in water, have the highest probability of getting reflected and re-emitted back out. As it stands, the global average albedo (the technical term for reflectivity) of our planet is 0.30, meaning 30% of the incident light gets reflected back into space. However, an awful lot of this reflectivity is due to cloud cover and stores of highly reflective ice. If the Earth were covered entirely in a deep-water ocean, our albedo would be just 0.11. The ocean, overall is actually an excellent absorber of sunlight!

The sky and ocean aren’t blue because of reflections at all; they’re both blue, but each one independently of the other, and due to their own physical properties and how they interact with light. If you took our oceans away entirely, a human on the surface would still see blue skies, and if you managed to take our skies away (but still somehow maintained the oceans of liquid water on our planet’s surface), our planet would still appear blue from faraway in space.

For the skies, the blue sunlight scatters more easily, and comes to us indirectly from where sunlight strikes the atmosphere as a result. For the oceans, longer-wavelength visible light gets absorbed more easily, so the deeper they go, the darker bluer the remaining light appears. Blue atmospheres may be common for planets, as Uranus and Neptune both possess them, too, but we’re the only one we know of with a blue surface. Perhaps when we find another world with liquid water on its surface, we’ll discover that we aren’t so unique after all, and in more ways than we even presently realize!

This article was first published in May of 2022. It was updated in August of 2026.

This article is featured on Big Think.