The Prism Effect

Most of us, upon learning that the skies and oceans are blue, hear the myth that “one is blue because it reflects the other.” This is simply not true.

The sky is blue because of how atmospheric scattering works, sending blue light in a variety of directions more effectively than red light, causing blue to arrive at our eyes from a variety of directions.

The oceans, meanwhile, preferentially absorb certain colors more than others, so as you go to deeper depths, the blue light is the last color that remains. Here’s the science of how both work.

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.

Regardless of where the Sun is in the sky, the color of the sky toward the zenith (directly overhead) is a much darker blue, while the sky toward the horizon is a lighter, brighter cyan color. This is due to the larger amount of atmosphere, and the larger amount of scattered light, that is visible at low angles on the sky.

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

I hope you get this right

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.

Through the vacuum of space, all light, regardless of wavelength or energy, travels at the same speed: the speed of light in a vacuum. When we observe light from a distant star, we are observing light that has already completed that journey from the source to the observer.

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.