The captivating colour of the sky has been a source of wonder and curiosity for generations.  The Rayleigh scattering explains why the sky is blue. It is a phenomenon in physics that describes the scattering of light or electromagnetic radiation by particles or molecules that are much smaller than the wavelength of the light. It is named after the British scientist Lord Rayleigh, who first described this type of scattering in the 19th century.

When light travels through a medium, such as the Earth’s atmosphere, it can interact with particles, such as molecules of nitrogen and oxygen, as well as other small aerosols like dust and water droplets. These interactions cause the light to scatter in various directions, leading to the phenomenon known as Rayleigh scattering. Shorter wavelengths are scattered much more effectively than longer wavelengths, according to Rayleigh. Blue and violet light have shorter wavelengths than red and yellow light, and hence are scattered more effectively.

The scattering of shorter wavelengths is also more efficient at shorter path lengths. A shorter path length refers to the distance that the sunlight has to travel through the Earth’s atmosphere before reaching our eyes. The higher the sun is in the sky, the shorter the path sunlight travels in the atmosphere compared to when the sun is lower on the horizon during sunrise or sunset.

During the daytime, when the sun is high in the sky, sunlight enters the Earth’s atmosphere and encounters these small particles. The shorter
wavelengths of blue and violet light are scattered in all directions, while the longer wavelengths of red and yellow light are less scattered and
continue to travel through the atmosphere more directly. As a result, when we look up at the sky during the day, we see a higher concentration of scattered blue. The efficiency of a shorter path also leads to a higher concentration of scattered blue light reaching our eyes. Hence when we look up into the sky, we see an intense blue covering us during the day.

Why is the sky not violet?

The sky is not violet because our eyes are more sensitive to blue light than to violet light, and the Earth’s atmosphere scatters blue light more
effectively than violet light. As sunlight enters the Earth’s atmosphere, it contains various colours of light with different wavelengths, ranging from violet to red. Violet light has the shortest wavelength among visible colours, and blue light has a slightly longer wavelength. When sunlight passes through the atmosphere, it undergoes Rayleigh scattering, where shorter wavelengths (blue and violet) are scattered more effectively by the molecules and particles in the atmosphere.

While both violet and blue light are scattered, blue light is scattered more efficiently due to its longer wavelength compared to violet.
Additionally, our eyes have three types of colour receptors, called cones, that are most sensitive to blue, green, and red light. The sensitivity of our eyes to violet light is lower compared to blue light. This means that when we look at the sky, the scattered blue light is more easily detected by our eyes, and as a result, we perceive the sky as blue.

Is the sky blue on other planets?

The colour of the sky on other planets in our solar system can vary significantly due to differences in their atmospheres, compositions, and
environmental conditions. The sky on Mars appears reddish or pinkish during the daytime. This is because Mars has a thin atmosphere composed mostly of carbon dioxide with traces of other gases. The Martian dust and fine particles in the atmosphere scatter sunlight, and the scattering is more effective for shorter wavelengths, giving the sky a reddish hue. The sky on Venus is mostly shrouded in thick clouds of sulfuric acid and other aerosols. These clouds scatter sunlight in all directions, creating a brilliant white appearance to the sky. Neptune also appears blue like Uranus, primarily due to the presence of methane in its atmosphere, which
scatters blue light.

The mesmerizing blue colour of the sky has captivated human curiosity for centuries, and now, we understand the fascinating phenomenon responsible for this enchanting display. Beyond our own planet, we find a kaleidoscope of colours that vary vastly due to unique atmospheric compositions and environmental conditions. The enchanting symphony of colours that grace the celestial canopy continues to inspire awe and wonder, inviting us to embrace the mysteries of the universe with open hearts and curious minds.