Light exhibits the phenomenon of refraction when it passes from one optical medium to another with a different density. for instance, when light enters water from air, it bends towards the denser medium (water). Similarly, earths atmosphere has varying densities that decrease exponentially with altitude, causing sunlight entering the atmosphere to refract.

During the day, sunlight hits the atmosphere almost vertically, so refraction is minimal. However, during sunset, sunlight travels horizontally through the atmosphere. The denser air layers closer to earths surface bend the light beams towards earth, and the light follows a curved path due to atmospheric refraction. 

This effect explains why we continue to see the sun even after it has geometrically set below the horizon- what we observe is the apparent sunset.

Sunlight

Sunlight is composed of white light, which is a combination of all visible wavelengths. Click here to learn more about electromagnetic waves. When light enters the atmosphere, it separates into its constituent colors because different wavelengths refract by different amounts. 

From the visible part of the spectrum, red light has the longest wavelength and the lowest energy, so it refracts the least. Blue light, with its shorter wavelength and higher energy, refracts more significantly.

Raleigh scattering

As sunlight passes through the thick layers of atmosphere during sunset, Raleigh scattering occurs. Rayleigh scattering is more effective for shorter wavelengths, like blue, causing these colors to scatter out of the line of sight. Only the linger wavelengths, like red and orange, reach our eyes, giving the sunset its characteristic hues.

During the day, the sky occurs blue because because shorter wavelengths, like blue, are scattered in all directions by molecules in the atmosphere, so the scattered light that reaches our eyes is predominantly blue, giving the sky its color.

Rainbows

Rainbows form when sunlight is refracted and reflected through water droplets in the atmosphere. Each color in the rainbow bends by a different amount. Colors with shorter wavelength appear on top of a rainbow because it is curves more strongly.