Sunlight and the Spectrum

Sunlight, or white light, is composed of a spectrum of colors, often remembered by the acronym VIBGYOR:

V: Violet

I: Indigo

B: Blue

G: Green

Y: Yellow

O: Orange

R: Red

Each of these colors corresponds to light waves of different wavelengths, with violet and blue light having shorter wavelengths and red light having longer wavelengths.

Interaction with the Atmosphere

When sunlight enters the Earth’s atmosphere, it interacts with molecules and small particles in the air. This interaction causes the light to scatter in different directions. The phenomenon responsible for this scattering is known as Rayleigh scattering.

Since blue light has a shorter wavelength than most other colors in the spectrum, it is scattered in all directions by the gases and particles in the atmosphere. Even though violet light is scattered even more than blue light, our eyes are more sensitive to blue light, and some of the violet light is absorbed by the upper atmosphere. As a result, the sky appears predominantly blue to us.

Daytime Sky

During the day, when the Sun is high in the sky, the shorter blue wavelengths are scattered in all directions, making the sky appear blue.

Sunrise and Sunset

At sunrise and sunset, the Sun’s light has to pass through a greater thickness of the Earth’s atmosphere. This means that even more of the shorter wavelengths (blue and violet) are scattered out of our line of sight, leaving the longer wavelengths (red and orange) to dominate. This is why the sky appears red and orange during these times.