Colours of the Sky and the Sea

Why does the sky appear blue? What is the reason for the spectacular red hues observed during sunrise and sunset? And what makes the ocean blue? These are some of the questions people ask whenever they observe the beauty of a magical sunset or gaze into the blue expanse of the ocean. What is the scientific reason for the incredible colour of the sky and the sea?

The visible light that enables us to see is made up of seven colours of the rainbow. Each of the colours has a specific wavelength, which is the distance between two successive crests of the light wave. Blue light has the shortest wavelength while red light has the longest.

What happens when sunlight enters the earth’s atmosphere? Upon entering the sky, sunlight is scattered by the particles present in the atmosphere. The earth’s atmosphere is predominantly made of nitrogen and oxygen molecules, which are smaller than the wavelength of light. These molecules absorb light and scatter the shortest wavelength (blue) much more than the others. Since blue light is scattered the most, the sky appears blue. This phenomenon is known as Rayleigh scattering, where light of a certain wavelength is scattered by particles of a much smaller wavelength.

However, the sky is red during sunrise and sunset. During these times, when the sun is on the horizon, light has to travel a longer distance through the atmosphere. Blue light is scattered much before sunlight enters the lower atmosphere, leaving only the red (and pink) light to reach our eyes. That is why, sunsets paint the sky spectacularly in red and pink.

Just after the Earth was formed, the atmosphere was totally different, with carbon dioxide, methane and hydrogen sulphide instead of nitrogen and oxygen. At that time, the colour of the sky was an eerie orange rather than blue. The differences in chemical composition of the atmosphere and much lesser scattering of blue contributed to the orange sky. On Mars too, the sky is reddish-pink due to the high concentration of iron particles in its atmosphere. In fact, the sunsets of Mars are blue, while the usual colour of sky is red, the exact opposite of that on Earth!

So why is the ocean blue? Many people assume that since the ocean sits right under the sky, it just reflects off the blue light from the sky. Though the sea does reflect certain amount of light from the sky and it is partially correct, that is not the complete answer. The ocean is blue mainly due to absorption of light, not scattering. No matter what colour the sky may be, water is always blue.

Let us consider water in a glass. It appears colourless, but water absorbs red, yellow and green light more than blue light. This effect increases with the volume of water. A swimming pool appears blue, while the ocean appears bluer. The ocean is blue mainly from absorbing light, rather than reflecting it. Since blue light is not absorbed by water, the ocean appears blue, giving the Earth its beautiful blue colour. Some animals have evolved to take advantage of water’s blue colour. Since red light is absorbed by water, animals with red skin look black in deep water, making them very difficult to spot.

However, there are certain places where the ocean is of a slightly different colour. For example, the colour of water might be turquoise or tinged with green. This is due to the presence of algae and other photosynthetic organisms called phytoplankton, which give a greenish tinge to the water. Sometimes, the presence of silt, mud and dirt can make the water look brownish (near the mouths of rivers).