The sky appears blue due to a phenomenon called Rayleigh scattering. When sunlight travels through the Earth’s atmosphere, it collides with gas molecules. These molecules scatter the light in all directions, but they scatter shorter wavelengths (blue light) more strongly than longer wavelengths.
This selective scattering means that:
- Blue light is scattered more throughout the atmosphere
- Red and yellow light typically passes straight through
- Our eyes see the scattered blue light from all directions in the sky
This is also why the sky appears more richly blue when looking straight up (where the atmosphere is thickest) compared to the horizon.