Click for the Filipino/Tagalog version of this article, Bakit Kulay Asul Ang Langit?
Why is the sky blue?
The sky is blue because of three things:
1. Light from the sun
2. Particles in our atmosphere
3. The fact that you’re human.
Let’s start with how sunlight interacts with particles in the sky. I’m not talking about dust or water droplets or other big particles, which are sometimes mistakenly used to explain the sky’s blueness. Instead, it’s the very small particles in our atmosphere that make the sky look blue. I’m talking about molecules of the air itself, mostly molecular oxygen and nitrogen -- O and N . These molecules are super-abundant in our atmosphere and so small that they’re even smaller than wavelengths of visible light!
Visible light consists of a range -- or spectrum -- of different wavelengths. Light with the longest wavelengths are at the red end of the spectrum, and light with the shortest wavelengths are at the blue and violet end. Since the blue wavelengths of light are shorter, they’re more likely to strike those tiny molecules of oxygen and nitrogen more often, and when they do, they get scattered in all directions. This effect is known as Rayleigh scattering, because it was first described by John William Strutt, aka the Third Baron Rayleigh, who published his mathematical proof that blue wavelengths of light in the atmosphere are scattered 16 times more than red wavelengths. But violet light has the shortest wavelength of all visible light, and it’s scattered even more strongly by tiny particles -- so why isn’t the sky purple? Well, this is where being human comes into play. Our human eyes perceive the middle of the spectrum most sharply which is why blue is way easier to see than violet. So even though violet light is being scattered around like crazy, we just don’t detect it as well as we detect the blue. And that’s why the violet band of the rainbow is often the hardest for us to make out.
