Why Is the Sky Blue?

Looking out of the window on a clear day, you will see a vivid blue sky. Look again at sunset, and the colours change to orange and red. The blue is not a reflection of the ocean, as is sometimes assumed. It is caused by the way sunlight is scattered in our atmosphere.

Explaining Why the Sky Appears Blue

We often think of the Sun as a glowing yellow orb in the sky. In reality, it emits white light containing all the wavelengths of visible light. Different wavelengths correspond to different colours. Red light has the longest wavelength (about 700 nm) and violet the shortest (about 380 nm), with blue just above violet (about 450–495 nm). When sunlight enters the atmosphere, the different wavelengths interact differently with the molecules in the air.

In the late 19th century, the British physicist Lord Rayleigh showed that gas molecules in the atmosphere scatter shorter wavelengths of light much more strongly than longer ones. The scattering strength is inversely proportional to the fourth power of the wavelength, so blue light is scattered roughly six times more than red light, and violet light roughly nine to ten times more.

Blue light is scattered throughout the atmosphere in all directions, so it reaches our eyes from every part of the sky. Red and orange light are scattered far less and mostly travel through the atmosphere undisturbed. Seeing this scattered blue light from all directions gives us the blue sky.

Why Not Violet?

Violet light has an even shorter wavelength than blue light, yet the sky does not appear violet, for three main reasons. First, the Sun emits less violet light than blue light. Second, some violet light is absorbed in the Sun’s outer layers and in Earth’s atmosphere. Finally, human eyes are much less sensitive to violet light than to blue light. Combined, these factors make the sky appear blue rather than violet.

Why Are Sunsets Orange?

At midday the Sun appears white or pale yellow. At sunrise and sunset, however, it appears reddish-orange. This is because its light must travel through much more of the atmosphere at a low angle than it does at midday. Much of the blue light is scattered out of the direct beam, leaving mostly the longer wavelengths to reach our eyes. Dust and other particles in the atmosphere deepen the reddish colours by scattering light as well.

Simple questions such as this reveal the intricate physics that make up our world. So, next time you observe a bright blue sky, think about how our eyes, the atmosphere, and light work together to bring colour into our surroundings.