Why is the Sky Blue?
When looking up at the sky on a beautiful sunny day, we’ve all asked ourselves: “Why is the sky blue?” It’s a fascinating question because it helps us uncover lots of interesting science about the sun and sky. There are two main things at play here: what sunlight is made from and how it interacts with the Earth’s atmosphere.
Sunlight is a Spectrum of Colour
The Sun’s bright light looks white to us, but it actually contains all the colours of the rainbow. We can see this when we look at it through a prism.
So, where do all these colours come from? The energy from the Sun, in the form of light, travels in waves and each kind is a different length. It’s these different wavelengths that are behind these different colours. Purple and blue colours, for instance, have the shortest wavelengths (waves with very short distances between their peaks). Red and orange colours have the longest wavelengths (longer distances between their peaks).
Visible light, the kind we can see with our naked eye, is imperceptibly made up of all these rainbow colours.
So, if the white light from the Sun is made up of all these rainbow colours, why does the sky almost always look blue? That’s where the Earth’s atmosphere comes into play.
The Atmosphere – The Great Disruptor
Wrapped around the Earth, the atmosphere is made up of a thin layer of gases, mainly nitrogen and oxygen. This is where the air we breathe comes from. This layer also keeps the planet at the right temperature so we can live here comfortably.
In the daytime, when the Sun is high in the sky, its light travels through space and makes its way to Earth, and to us, in a direct line where the distance is the shortest. Sunlight collides with the nitrogen and oxygen molecules in our atmosphere and gets dispersed in every direction. The way the different waves (or colours) of sunlight are dispersed depends on their wavelength. Short wavelengths, like red, are refracted much less than longer wavelengths, like blue, which are refracted much more. So, when we look up at the blue sky, what we’re actually seeing are those blue wavelengths that have been dispersed more than the others across the atmosphere.
At night, when the white light of the Sun is not visible, the sky reflects no colour at all. That's why it looks black! If we lived on the Moon, the sky would always look black. That’s because its atmosphere is not like ours here on Earth. Sunlight does not interact with its atmospheric molecules the same way it does with the ones here on Earth.
Why is the Sky Sometimes Orange, Yellow, or Shades of Red?
In the morning or in the evening, the Sun is low on the horizon and its light has to travel further through the atmosphere. That means that its wavelengths collide with many more gas molecules.
How Come the Sky is Never Purple?
Even though the Sun emits all the colours of the rainbow, it doesn’t emit them all at the same strength. The Sun emits more blue wavelengths than purple ones, and some of these purple wavelengths get absorbed by the Earth’s upper atmosphere which is further away from us. This means that fewer purple wavelengths make it into our atmosphere to become visible to our eyes.
Want to know something else? Our eyes have evolved to see certain colours (or wavelengths) better than others. Our retinas have colour receptors that easily recognize the wavelengths associated with red, blue and green. That’s why our eyes will pick up blue more than they pick up purple.
So, the next time you look up at the sky on a beautiful afternoon, or the next time you enjoy a gorgeous sunset, you can say, “Look at those beautiful wavelengths splashed across the atmosphere!”
When you visit Explore, one of our permanent exhibitions, you’ll be able to play with light and refract it with prisms of different sizes and even send it in different directions with a series of mirrors!
