Why Sky Appears Blue?

Have you ever wondered  why  sky is blue  during the day time? This is because of the Rayleigh Scattering phenomenon which we will discuss below. 

The Basics Of  Light And Atmosphere 

Sunlight, or white light, is composed of a spectrum of colors, each with its own wavelength. When sunlight reaches Earth, it must pass through our atmosphere, which is filled with gases and particles. The size of the particles in the atmosphere is smaller than the wavelength of light, so they are more effective in scattering the light of smaller wavelength (blue end of spectrum). When the sunlight passes through the atmosphere, the fine particles scatter the blue light more strongly than red light. The scattered blue light enters our eye. Hence the light appears blue. It should be noted that the sky appears black to the passengers flying at higher altitude because scattering of light is not evident at such height due to the absence of particles.

Rayleigh Scattering:

Rayleigh scattering occurs when light interacts the particles that are much smaller than its wavelength. This type of scattering is  called Rayleigh Scattering named after the British scientist Lord Rayleigh, who first described it in the 19th century.

Sunlight consists of a range of colors, from violet and blue at the short-wavelength end to red and orange at the long-wavelength end. When sunlight enters the Earth's atmosphere, the shorter wavelengths (blue and violet) are scattered more than the longer wavelengths (red and orange).

Also the scattering effect is inversely proportional to the fourth power of the wavelength. In simpler terms, shorter wavelengths scatter much more efficiently than longer wavelengths.

Why the sky is blue not Violet?

Even though violet light is scattered even more than blue light, inspite of this our sky appears blue rather than violet due to the following reasons:

1. Sunlight Composition: The sun emits less violet light compared to blue light.

2. Human Vision: Our eyes are more sensitive to blue light and less sensitive to violet light.

3. Atmospheric Absorption: The upper atmosphere absorbs some violet light, preventing it from reaching our eyes.

Changing Colour Of  The Sky:

The colour of the sky changes throughout the day due to the angle of the Sun. At sunrise and sunset, the Sun and the sky appears red. Light from the Sun near the horizon passes through thicker layers of air and covers larger distance in the atmosphere before reaching our eyes. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun and the sky. However at the noon, the light from the Sun overhead would travel relatively shorter distance. So, it appears white as only a little of the blue and violet colours are scattered.