Why is the ocean blue? Water, sunlight and changing colours

Water changes the mixture of light

Sunlight contains many visible wavelengths. As it travels through water, those wavelengths do not all survive equally well: red light is absorbed more readily, while blue can travel farther. NOAA describes this selective absorption as the central reason for the ocean’s blue appearance. [1]

Depth matters because it changes how much water the light has passed through. A small, clear sample and a deep body of water can therefore look different without containing different blue substances. Blue water does not require blue dye.

Is the sea reflecting the sky?

Sometimes, yes. A surface reflection can make the same stretch of sea look different under a bright sky, grey clouds or sunset colours. But reflection alone cannot account for the blue light observed beneath the surface. Woods Hole Oceanographic Institution distinguishes this surface effect from what happens inside the water. [2]

When interpreting a photograph, ask whether you are mainly seeing a reflection or light returning from below. A single image cannot always separate the two.

Why does some seawater look turquoise, green or brown?

These mechanisms are described by Woods Hole. They are clues, not a colour-based test of water quality. [2] The table cannot tell you whether a particular beach is safe for swimming.

A useful way to compare two beach photographs

Before deciding that the water itself changed, compare the sky, viewing angle, water depth and visible sediment. Also check whether one image has much stronger colour editing. This is an observation checklist, not a laboratory measurement.

Blue colour and saltiness answer different questions. Our companion explanation of where ocean salt comes from follows dissolved minerals rather than light. The photosynthesis guide explains the role of pigments in capturing energy.