Why does the sea look blue?

KohakuThe sea is blue as far as the eye can see. Is it simply reflecting the color of the sky?

The answer in 30 seconds

In fact, the reason lies in the water itself.

The sea looks blue because water easily absorbs the reddish colors in sunlight, leaving the blue. The remaining blue light bounces around in the water and reaches our eyes. The surface does reflect the color of the sky, but the sea is still blue when you dive in, which shows that the water itself creates the blue color.

The deep blue Ionian Sea seen from a cliff, with the white wake of a boat — The Ionian Sea seen from the top of a cliff. Deep, clear seas look dark blue.Photo: Nathannn20 / CC BY-SA 4.0 / Wikimedia Commons

Water absorbs red light

Sunlight is a mix of many colors, from red to violet. According to the U.S. National Oceanic and Atmospheric Administration (NOAA), seawater easily absorbs the reddish colors in light and, like a filter, leaves the bluish colors behind. That is why the sea looks blue.

It's not just reflecting the sky

People sometimes say, "The sea is blue because it reflects the color of the sky." Woods Hole Oceanographic Institution explains that the surface of the sea does reflect the color of the sky. That is why the sea looks deep blue on sunny days and gray on cloudy days. But the blue does not disappear when you dive into the sea. The blue under the water is a color made by the water itself.

KohakuIt stays blue even when you dive in, because the water itself makes the blue color.

Colors fade away as it gets deeper

As light travels through water, the long-wavelength colors, red and orange, are absorbed first and almost disappear within a few meters. Next, yellow and green fade away, and the short-wavelength colors, blue and violet, reach the deepest. The remaining blue light bounces off tiny particles in the water, so the water looks blue.

Figure: How deep each color reaches

Bar lengths are only a rough guide to show the order.

Some seas look green or brown

The color of the sea is not always the same. According to NASA, seas with a lot of phytoplankton (tiny drifting plants) look greenish, because their chlorophyll absorbs red and blue light and gives back green light. Where soil washes in from rivers, the sea looks brownish, and where it is shallow with white sand or coral, it looks a bright, emerald-like blue.

Figure: When the color of the sea changes

Want to know more?

According to NOAA, very little light reaches below a depth of 200 meters, and no light at all reaches below 1000 meters.
Many deep-sea creatures have red or black bodies. At depths where only blue light reaches, a red body is actually harder to see.
Studying the color of the sea with satellites reveals things like the amount of phytoplankton, which helps scientists in their research.

KohakuWater absorbs the reddish light and leaves the blue light behind. That is the main reason the sea looks blue.

Quick quiz: test yourself

What is the main reason the sea looks blue?

Show the answer

Water absorbs reddish light
It reflects the color of the sky
The salt in seawater is blue

Water absorbs reddish light Water easily absorbs the reddish colors in sunlight, leaving blue light. The sea is still blue when you dive in, so the water itself makes the blue.

Sources

Why is the ocean blue?NOAA National Ocean Service
Why is the ocean blue?Woods Hole Oceanographic Institution
What Color is the Ocean?NASA

This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.