Derby and District Astronomical Society
The Journal of the Derby and District Astronomical Society
The Nature of Light and What Makes the Sky Blue
By Maurice Batchelor
We live on a planet quite different from any other planet in the Solar System, ideally suited to our requirements, colourful and magnificent and teeming with life. The main reason for this is its atmosphere and its position in relation to the Sun. At approximately 93,000,000 miles, its maintains a suitable range of temperatures for varying forms of life, however, the main topic of this article is to explain why the Earth has a blue sky, as distinct from the Moon, where the sky appears black. The reason for this is the way a beam of light behaves, having entered the structure of the Earth atmosphere.
To go more deeply into this, we must first examine the nature of light itself. A beam of light consists of a series of colours, not visible to the human eye. They are, red, green, blue, indigo (this is a deep blue) and violet, this makes up the complete spectrum. This can only be seen through an instrument known as a spectroscope. Nature, however, has its own way of demonstrating this in a rainbow, where the same principles apply, light entering the Earth’s atmosphere will encounter, and interact with, minute particles of dust and molecules. It also can also pass through matter, when this happens the light is said to be transmitted. A transparent media, such as a pane of glass, transmits light well. Another example of the way light behaves, is its ability to 'bounce off' an object. This is what is known as 'reflected light', and is the way the human eye determines the colour of an object. Light can also be absorbed by matter - absorbed light is not visible to the observer.
We must now consider what happens when a beam of light travelling through the void of space towards the Earth, enters the Earth’s atmosphere and encounters belts of dust and molecules. It is at this stage that an interesting development takes place. The constituent colours of light are selectively reflected, or, 'bounced off' these molecules and dust particles in all directions. This is known as 'scattering'. The colour mostly scattered in the case of the Earth’s atmosphere is blue. The question now arises – why blue? Why this colour of light in preference to all other colours that make up a beam of light? The composition of the atmosphere determines the nature of the scattering. If the particles that do the scattering are smaller in size than the wavelength of the light scattered, then blue will be the main colour scattered. Blue light, therefore, is selectively scattered more than any other colour. Blue light is in fact scattered about 10 times as much as red light. The process of selective scattering of blue light by very small particles is known as Rayleigh Scattering. To illustrate how this comes about see the diagram below.
Rayleigh Scattering. Diagram by the author.
Blue skies, however, are rather rare in the British Isles – the question arises then, why are clouds not blue? This is because the particles scattering the light within the clouds are spherical in shape and are NOT smaller than the wavelength of the light they interact with. Also, wavelengths are scattered equally, blue not selected over any other colour. This is known as Mie Scattering. Clouds consist mainly of water droplets and ice crystals which correspond with the size and shape required for the Mie Scattering. The result being a blend of all colours of light which come across as white - hence white clouds.
As previously mentioned, the sky as seen from the Moon, for instance, appears black, with a brilliant Sun shining down. This is because the Moon retains no atmosphere. It is not necessary to travel all the way to the Moon, about a quarter of a million miles to experience this effect. At about 100 miles above the Earth’s surface, there is essentially, due to the thin atmosphere, no scattering of light. The sky appears black, the Sun more brilliant and the Moon, stars and planets all visible together.