A blue sky, a red sunset, a rainbow and a deep blue sea may look like completely different phenomena, but they can all be understood by studying how light interacts with matter. Visible light is part of the electromagnetic spectrum, and different colours correspond to different ranges of wavelength. When light encounters air, water or other materials, it can be reflected, refracted, scattered or absorbed. These different interactions help create many of the colours we see in nature.

The daytime sky appears blue mainly because of a process called Rayleigh scattering. Sunlight contains a range of visible wavelengths. As sunlight travels through Earth's atmosphere, it interacts with molecules in the air. Shorter visible wavelengths are scattered much more strongly than longer wavelengths. Blue light is therefore scattered across the sky and reaches our eyes from many different directions, making a clear daytime sky appear predominantly blue.

At sunset, sunlight travels through a longer path in the atmosphere before reaching an observer. Much of the shorter-wavelength blue light is scattered away from the direct path. The light arriving directly from the direction of the setting Sun therefore contains a greater proportion of longer visible wavelengths, producing familiar orange and red colours. Dust, aerosols and other particles in the atmosphere can also influence the appearance and intensity of a sunset.

Rainbows are produced by a different combination of optical effects. When sunlight enters a spherical raindrop, it changes direction because its speed changes as it enters the water. This bending is called refraction. Different wavelengths are refracted by slightly different amounts, producing dispersion. Light can then reflect from the inside of the raindrop before being refracted again as it leaves. Because different wavelengths emerge at different angles, an observer can see the separated colours that form a rainbow.

The colour of the sea has another explanation. Pure water absorbs different wavelengths of visible light by different amounts. Longer wavelengths towards the red part of the spectrum are absorbed more strongly than blue wavelengths, helping large volumes of relatively clear water appear blue. Light reflected from the sky can affect the appearance of the surface, but it is not the complete explanation for why the ocean is blue. Sediment, algae, depth, the sea floor and the angle of illumination can also make seawater appear green, turquoise, brown or other colours.

These examples show why similar colours do not necessarily have the same physical cause. The blue sky involves scattering, a rainbow involves refraction, dispersion and internal reflection, and the colour of deep water is strongly influenced by selective absorption. By studying wavelength and the different ways light interacts with matter, scientists can explain some of the most familiar and spectacular colours in the natural world.