WHY IS THE SKY BLUE?
The sky is a beautiful artist, normally covering itself in sky blue. In horizon and sunset, it paints itself into the beautiful orange that sets the background. In the night, it turns dark blue and calm, with stars beautifying the scenery. However, have you ever wondered how the sky changes its colour every time? Why is it so colourful?
Index of contents:
- Discuss about scattering of light
- Different forms of scattering of light
- Physical processes that change visible appearance
- The Atmosphere
- How scattering of light occurs
- How does scattering of light affect the sky’s colour
- Role of Oxygen and Nitrogen in the air
- Amount of oxygen and nitrogen in the air
- How oxygen and nitrogen affect light scattering
- How does oxygen and nitrogen affect sky’s colour
- Content of Water vapour
- Content of water vapour
- How does water vapour contribute to the scattering of light
- What happens when there are a lot of dust particles
- Dust particles in the air
- How does dust particles affect scattering of light
- What happens when there are a lot of dust particles?
- Other examples of scattering of light
- Mie Scattering
- Brillouin Scattering
- Tyndall Scattering
- Resources
1 - Discuss about scattering of light
Different forms of scattering of light
At first, what is the scattering of light?? A brief explanation is that the spreading or scattering of light by particles in a mixture is called the scattering of light. There are many forms of scattering of light. Forms of elastic light scattering are Rayleigh scattering and Mie scattering. Inelastic scattering includes Brillouin scattering, Raman scattering, inelastic X-ray scattering and Compton scattering. These are the several electromagnetic scattering that are distinct to have conventional names.
Physical processes that change visible appearance
There are two major physical processes that contribute to the visible appearance of most objects, one being the scattering of light and the other is absorption of light. We all know that how the colours black and white is derived, in simple theory is that one matter that absorbs any light is black, while one matter that scatters any light is white.
The Atmosphere
The atmosphere on Earth contributes to the scattering of light. In space which is vacuum, there is no atmosphere to scatter the sun’s light. So on Earth, the atmosphere is the mixture of gas molecules and other particles that surround the Earth. Most common gases are Nitrogen and Oxygen, followed by Argon gas and water (which in the form of vapour, droplets and ice crystals), with other amounts of gases and solid particles like dust, ashes, pollens and salt form the ocean. Nitrogen makes up 78% of the atmosphere while oxygen makes up the 21%. The remaining 1% is water vapour dust, and other small particles.
These are what that makes up the atmosphere, and of course, the composition of the atmosphere also depends on the location. In tropical areas there may be more water content in the air. In an area where a volcano that just erupted may result in a lot of dust and ashes in the air. In urban areas where pollution is heavy, the atmosphere may contain a high amount of dust particles and carbon dioxide. The atmosphere is denser when it is near the Earth, and it gradually thins out as it goes higher and higher.
How scattering of light occurs
We know that light travels in a straight line as long as nothing disturbs it. As the atmosphere contains gas, particles and molecules, they have become ‘disturbers’. Then as the light bumps into these ‘disturbers’, which are particles and molecules, scattering occurs and how the light is scattered depend on wavelengths and the particle it ‘bumps’. How the light is scattered can be calculated using complex mathematics and physics.
Picture 1
Dust particles and water contents are much bigger than the wavelength of visible light. As such when light hits into these larger particles, it gets bounced off or reflected in different directions. No colours were affected as the light is reflected in the same way. However, when light hits a gas molecule, the gas molecule is smaller than the wavelength of visible light, and thus emits a different light colour instead. When light hits these gas molecules, some of the light gets absorbed. Then the molecule releases the light in different directions, and the colour emitted is the colour that was absorbed. All colors can be absorbed, but higher frequency colours like blue are absorbed more often than lower frequency colours like red. This process is known as Rayleigh scattering.
Dust particles and water contents are much bigger than the wavelength of visible light. As such when light hits into these larger particles, it gets bounced off or reflected in different directions. No colours were affected as the light is reflected in the same way. However, when light hits a gas molecule, the gas molecule is smaller than the wavelength of visible light, and thus emits a different light colour instead. When light hits these gas molecules, some of the light gets absorbed. Then the molecule releases the light in different directions, and the colour emitted is the colour that was absorbed. All colors can be absorbed, but higher frequency colours like blue are absorbed more often than lower frequency colours like red. This process is known as Rayleigh scattering.
How does scattering of light affects the sky’s colour
The sky appears blue because of Rayleigh scattering. As mentioned in previous paragraphs, Rayleigh scattering is where higher frequency colours are absorbed more often than lower frequency colours, and the molecules that release the light has the colour which is absorbed. When sun light, which is white, enters the atmosphere, light hits the gas molecules which are present in the atmosphere, causing Rayleigh scattering. As Rayleigh scattering is where higher frequency colours like blue are absorbed more often and released, the sky thus appears blue. In sun sets, the light must travel farther through the atmosphere before it gets to us. More of the light is reflected and scattered. As less light reaches us directly, the sun appears less bright. The colour of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes. (Refer to Picture 2)
2 - Role of oxygen and nitrogen in air
Amount of oxygen and nitrogen in air
We have just known that atmosphere is composed of gases and particles. The most common gases were oxygen and nitrogen. As mentioned before, Nitrogen makes up 78% of the atmosphere while oxygen makes up the 21% (Refer to Picture 1). The remaining are the particles and water content. For now, we will discuss how nitrogen and oxygen affects light scattering in air.
How Oxygen and Nitrogen affect light scattering
Oxygen and Nitrogen particles are most effective in scattering higher frequency and short wavelength. Atmospheric nitrogen and oxygen scatter violet light most easily, followed by blue light, green light etc. White light contains colours Red, orange, yellow, green, blue, indigo and violet (ROYGBIV), also known as the rainbow colours. As white light from the sun passes through Earth’s atmosphere, the high frequency light (Blue, Indigo, Violet) become scattered by atmospheric particles, which are mostly nitrogen and oxygen, while low frequency light (Red, Orange, Yellow) are most likely to pass through the atmosphere without alteration in their direction.
How Nitrogen and Oxygen affects the sky’s colour
As Nitrogen and Oxygen affect the scattering of light as mentioned, the scattering of light (BIV) illuminates the sky on the BIV end of visible spectrum. Although violet light is more easily scattered by the particles, our eyes are more sensitive to blue light, which is why the sky appears blue to us.
Picture 2
3 - Content of water vapour
Content of water vapour
Water vapour is the gas phase of water. Under typical atmospheric conditions, water vapour is constantly generated through evaporation, boiling of water or sublimation of ice, and is removed through condensation. Water vapour content is the content of water vapour in the air. Atmospheric water vapour content is expressed using various measures like water pressure, relative humidity, mixing ratio, dew point temperature and specific humidity.
How does water vapour contribute to the scattering of light
The atmosphere contains gases and of course, water vapour. Water vapour is one of the ‘disturbers’ that reflect light in different directions. Water vapour affects how light scatters in the air. For example, when there is a high water vapour content in an area, it might result into fog. Fog obstructs vision. Simple as that. Water absorbs longer wavelength of light, and the same property is applied in water vapours. Dust, pollen, smoke and water vapour are common causes of Mie scattering which tends to affect longer wavelengths.
4 - What happens when there are a lot of dust particles
Dust particles in the air
What is dust? Dust consists of particles from the atmosphere that arise from various sources. Dust can be soil dust lifted by wind, caused by volcanic eruptions, or even from pollutions. Dust in human environments includes plant pollen, hair, human cells and other materials that can be found in that particular human environment. Dust particles actually float around in the air, but they are very small in number and sizes, which is why we occasionally get dust in our eyes.
How does dust particles affect light scattering
The atmosphere also includes dust and particles. Dust is one of the ‘disturbers’ as well, a dust particles are normally larger than wavelength of visible light, as such they reflect the light instead. Dust particles are like solids floating in the air, and in the law of reflection, solids reflect light.
What happens when there are a lot of dust particles?
When there are a lot of dust particles, the vision is blurred as light cannot travel entirely into our eyes. Dust particles are like a solid broken into very minute pieces, so light that it could be lifted by wind. When there are a lot of dust particles, the light that passes through it will reflect to one another particles, and when this happens, we cannot get a clear image of what is behind the cloud of dust particles. Such examples are like haze, sand storm etc.
In sunsets, the particles reflect light in all directions. Then, as some of the light heads towards us, different amounts of the shorter wavelength colours are scattered out. We see the longer wavelengths, and the sky appears red, pink or orange.
In sunsets, the particles reflect light in all directions. Then, as some of the light heads towards us, different amounts of the shorter wavelength colours are scattered out. We see the longer wavelengths, and the sky appears red, pink or orange.
5 - Other examples of scattering of light?
Mie Scattering
Mie scattering occurs when the particles in the atmosphere are the same size as the wavelengths being scattered. Dust, pollen, smoke and water vapour are common causes of Mie scattering which tends to affect longer wavelengths. Mie scattering occurs mostly in the lower portions of the atmosphere where larger particles are more abundant, and dominates when cloud conditions are overcast.
(Wikipedia – Mie Theory in “Atmospheric Science”)[7]Brillouin Scattering
Brillouin scattering, named after Léon Brillouin, occurs when light in a medium (such as air, water or a crystal) interacts with time dependent optical density variations and changes its energy (frequency) and path. The density variations may be due to acoustic modes, such as phonons, magnetic modes, such as magnons, or temperature gradients.
(Wikipedia – Brillouin scattering)[9]Tyndall scattering
The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension. It is named after the 19th century physicist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is scattered much more strongly than red light. An example in everyday life is the blue colour sometimes seen in the smoke emitted by motorcycles, particularly two stroke machines where the burnt engine oil provides the particles.
(Wikipedia – Tyndall Effect)[10]Resources
[1] http://www.sciencemadesimple.com/sky_blue.html
[2] http://en.wikipedia.org/wiki/Scattering
[3] http://en.wikipedia.org/wiki/Rayleigh_scattering
[4] http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/opt/mch/sct.rxml
[5] http://en.wikipedia.org/wiki/Water_vapor
[6] http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html
[7] http://en.wikipedia.org/wiki/Mie_theory
[8] http://en.wikipedia.org/wiki/Dust
[9] http://en.wikipedia.org/wiki/Brillouin_scattering
[10]http://en.wikipedia.org/wiki/Tyndall_effect
Pie Chart created using Microsoft Excel
Pictures are drawn using Microsoft Paint
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