Hues across the horizon

Review

Chinmaya KV

Coastal sites exhibit red coloured skies in the mornings and at sunsets.
Picture taken in Pondicherry.
Photo Credit : Chinmaya KV

  An entertaining read about simple observations to deep ruminations on the colors in the sky. The article goes down the memory lane from the simple delights of watching a rainbow to the contributions of Prof. C.V Raman, the first Indian scientist who won the Nobel Prize in 1930 for explaining the scattering of light by molecules.



  When you wake up early in the morning and look at the sky, it appears as a spectacular blend of red, yellow and blue. Afterward, in the day, the sky gradually becomes blue and dazzling white follows at noon. In the evening, the same trend occurs in reverse order until it plunges into the darkness of the night.

  No doubt, the fascinating display of colours inspired the impressionists to paint their canvases. Scientists got curious to unravel the mystery behind it as well. They understood that the appearance of the colourful sky is due to three simple interplaying factors. Firstly, the sunlight which has different wavelengths of electromagnetic radiations. Secondly, the particles in the Earth’s atmosphere that disperse the sunlight to varying amounts and directions. Finally perception of the light by the human eyes.

  But before getting into the elaborate discussion, let me invite you to recall the joys of watching rainbows. A fascinating phenomenon happens in nature when sunlight shines from behind the observer on the water droplets in the air at a low altitude range. During the course, the light travels from one medium (Air) to another (Water) at some angle. As a result, the light beam bends and splits up into its constituent seven colors. Each color has its frequency and wavelengths while violet has the shortest red longest wavelength. In 1869, Scientist Rayleigh figured out the hidden mathematical rule behind the extent of the scattering of light. He showed that the intensity of the scattered light ( energy flowing per unit area per unit time) is inversely proportional to the fourth power of its wavelength. In simple words, the lesser the wavelength the more will be the intensity.

White light disperses into its constituent wavelengths upon passing through a prism White light disperses into its constituent wavelengths upon passing through a prism Comparison of wavelength, frequency and energy for the electromagnetic spectrum Comparison of wavelength, frequency and energy for the electromagnetic spectrum. (Credit: NASA's Imagine the Universe)

  When sunlight shines towards the air molecules, they absorb and reemit the light in all possible directions. Colors depend on what molecules are present in the atmosphere. Unlike Earth, the blue colored sky is not seen in the daytime on other planets. Recently, NASA’s rovers captured the image of the sky of planet Mars. The Martian sky takes on an orange or reddish hue during the daytime. But in the evening, it appears as blue-gray1. The particles present in the atmosphere of Mars disperse light differently than those present in the Earth. The atmosphere of Mars consists of carbon dioxide along with the dust particles.

  Our earth is predominantly composed of gases like nitrogen (78%), oxygen (21%) etc. by volume. Trace amounts of water vapor and dust particles are also present. Small size particles effectively scatter light of low frequencies like violet and blue. The abundant amount of oxygen present in the atmosphere of Earth scatters blue light the most. As a result, the sky looks blue during the day.

Rayleigh Scattering causes us to perceive a blue sky during daytime and a red sky at sunset. Rayleigh Scattering causes us to perceive a blue sky during daytime and a red sky at sunset. Image Source: Principles of Remote Sensing (Tempfli et al.2)

  But why do most of the time the sky appears blue instead of violet and indigo? It is notable that the range in the spectrum of light emission from the sun is not constant at all wavelengths. Also, human eyes are biological cameras. They can detect colors wonderfully within the visible range in the electromagnetic spectrum (Figure 1) like the bees have the ability to detect UV range frequencies.(fig 2) Human eyes are made up of two types of cells called cones and rod cells. Each of them are sensitive to light of different wavelengths. Our eyes respond more strongly to blue, cyan, and green wavelengths of light than they do to violet. Even though if there is more violet light, it is not enough to overcome the strong blue signal our brains deliver.3.

Bees capture minute details of a flower because of ultraviolet vision unlike human beings Bees capture minute details of a flower because of ultraviolet vision unlike human beings

  What happens during the sun set? The sunset will appear either yellow or yellowish red when the air is clean but appears redder in the presence of natural or synthetic pollutants. It is because some of the blue light has scattered away hence leaving behind a pure red hue to filter through. Sunsets over the sea may also be orange due to the presence of salt particles in the air.They are effective Tyndall scatterers. While Rayleigh scattering involves particles that are far smaller than the wavelength of light, Tyndall scattering is done by bigger particles called colloids. Hence Tyndall scattering is more intense than Rayleigh.

IISER Kolkata skies are often a delight to watchPicture Credits - Fazal Kareem Lilac blue is an unusual shade observed at dusksPicture Credits - Shrestha Chowdhury

IISER Kolkata skies are often a delight to watch. Lilac blue is an unusual shade observed at dusks.

  Some people believe that the ocean looks blue due to the reflection of the sky. Prof. CV Raman established that the blue color of the seas occurs due to the scattering of light by the water molecules. People now know it as Raman scattering. Raman scattering involves a change in the frequency of incident light upon deflection, unlike Rayleigh scattering. The frequency shift is characteristic of the molecule thus can be used for the detection of unknown compounds. Prof CV Raman won the Nobel prize in 1930 for the discovery of Raman effect. He was the first Indian to win a Nobel prize in Science.

CV Raman This Year 2020, 131st birth anniversary of Prof. CV Raman who won the first Asian Nobel Prize for Physics in 1930 for discovering the Raman Effect4
Illustration by - Naman Agrawal


Bibliography

  1. "Why Is the Sky Blue? - NASA Space Place." Accessed 21 Oct. 2020.
  2. "Principles of Remote Sensing." Accessed 21 Oct. 2020.
  3. "Why The Sky Is Blue, According To Science - Forbes." 8 Sep. 2017, Accessed 24 Oct. 2020.
  4. "CV Raman Birth Anniversary: Did you know ... - Catch News." 7 Nov. 2019, Accessed 23 Nov. 2020.

Chinmaya KV did his Masters in Physics and now he is a long term fellow at Open Academic Research group, CCMB, Hyderabad. His predominant interest is in science communication through writing, interviewing researchers and photography for the public. As a student of science, he has observed that quick comprehension of scientific information is not easy, and varies vastly between audiences and individuals, depending on interest and skill-level.

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