It’s a question most of us first asked as kids: why is the sky blue? And while the answer might sound simple, it’s actually rooted in interesting science. The colour of the sky is the result of how sunlight interacts with Earth’s atmosphere. Don’t fully understand it? Don’t worry, in this article we explain everything as we take a closer look at light, air, and a phenomenon known as Rayleigh scattering.
Sunlight isn’t just one colour
To start, it’s important to understand that sunlight, also called white light, isn’t actually white. It’s made up of all the colours of the rainbow—red, orange, yellow, green, blue, indigo, and violet. Each of these colours travels in waves, and each wave has a different length. Red light has the longest wavelength, while blue and violet have much shorter wavelengths.
When sunlight enters Earth’s atmosphere, it collides with tiny molecules of oxygen and nitrogen. This is where the magic happens.
How Rayleigh scattering makes the sky blue
As sunlight travels through the atmosphere, the shorter wavelengths of light—blue and violet—are scattered in all directions by the air molecules. This scattering process is called Rayleigh scattering, named after the 19th-century scientist Lord Rayleigh, who first described it.
But if blue and violet light are scattered the most, why doesn’t the sky appear violet? The answer lies in how our eyes perceive colour. Our eyes are more sensitive to blue light than violet, and there’s also less violet light in sunlight compared to blue. As a result, the sky appears predominantly blue to us.
Why the sky changes colour at sunrise and sunset
The sky doesn’t always stay blue, and its shifting hues at sunrise and sunset offer another glimpse into the science of light. During these times, the sun is lower on the horizon, so sunlight has to pass through a thicker layer of atmosphere to reach our eyes.
As it travels, the shorter wavelengths—blue and violet—are scattered so much that they don’t reach us. What’s left are the longer wavelengths like red and orange, giving the sky its warm, fiery tones.
This effect is enhanced by particles like dust and pollution in the atmosphere, which scatter light differently and can make sunsets even more vivid.
A glimpse into the universe
The colour of the sky is more than just a beautiful backdrop—it’s a reminder of how light and matter interact on a fundamental level. It’s also why the skies of other planets look different. For example, Mars has a thin atmosphere mostly made of carbon dioxide, which scatters light differently. As a result, the Martian sky often appears reddish or even pinkish.
The next time you look up at a clear blue sky, you’ll know there’s a lot more going on than meets the eye. It’s a blend of physics, biology, and natural wonder—just one of the many everyday mysteries that science helps us uncover.
