Everyone knows the sky is blue. Almost everyone has heard the word ‘Rayleigh scattering.’ Almost no one knows what it actually means.
Let me fix that.
What Is Light?
First, a quick foundation. Visible light is electromagnetic radiation — oscillating electric and magnetic fields propagating through space at 300,000 kilometers per second.
What makes red light red and blue light blue is the frequency of those oscillations. Blue light oscillates about twice as fast as red light. (Or equivalently, blue light has a shorter wavelength — about 450 nanometers vs. 700 nanometers for red.)
White light — sunlight — contains all frequencies mixed together.
What Happens in the Atmosphere
The atmosphere is full of nitrogen and oxygen molecules — tiny objects compared to the wavelength of visible light.
When light hits one of these molecules, the molecule absorbs the photon briefly, gets disturbed, and then re-emits a photon in a random direction. This is scattering.
Here is the key: molecules scatter short wavelengths (blue) much more than long wavelengths (red). Specifically, scattering intensity scales as the inverse fourth power of wavelength. Blue light has a shorter wavelength — so it gets scattered about 5–10 times more than red light.
So Why Is the Sky Blue?
When you look anywhere in the sky except directly at the sun, you are seeing scattered light. Light that bounced off molecules and reached your eyes from the side, from above, from everywhere.
Because blue light scatters more, the sky appears blue from every direction except directly at the sun.
And Why Are Sunsets Orange?
At sunset, sunlight travels through a much longer path of atmosphere to reach you. Over that long path, almost all the blue light has been scattered away. What remains — what reaches your eyes — is the red and orange that wasn’t scattered. Hence the orange sunset.
This is the same phenomenon, viewed from the other end.