Wednesday, June 5, 2019

Scattering of Light

Scattering of LightCallum LimContents1. Rayleigh divide1.1 Mid-day1.2 sunshinerise and sunset1.3 Night-time2. Mie scattering2.1 Clouds3. contaminant3.1 Light pollution3.2 Haze4. Personal reflections5. Conclusion6. Bibliography1. Rayleigh scatteringWhen a beam of readablesome travels by subdivisions which argon smaller than its wavelength, it is practicable these particles scatter the write down beam in a process kat oncen as Rayleigh scattering. This process is visible everyday whenever sunshine is scattered by the Earths standard atmosphere. As sun argus-eyed is composed of different visible wavelengths of light mainly Red, Orange, Yellow, Green, Blue, Indigo and Violet ROYGBIV non all sunlight has the same probabi illuminatedy of being scattered.It is possible to determine the amount of Rayleigh scattering that the different visible wavelengths in sunlight experience by calculating the intensity of the individual scattered wavelengths. The intensity of light scattered by small particles (I) depends on the intensity (I0) and wavelength () of the light point of reference, the distance of the light source from the particle (R), the scattering angle (), the refractive index of the particle (n), and the diameter of the particle (d). The relationship between these variables is illustrated in the following equation(Seinfeld Pandis , 2006)1.1 Mid-dayWhen the Sun is directly overhead, all the wavelengths of light originating from it travel the same distance from the Sun and done our atmosphere and ar scattered by the same medium (nitrogen and oxygen molecules). Hence, I0, R, , n and d remain constant and the above equation foot be simplified intoWith this equation, it can be seen that the intensity of scattered light is inversely pro chanceal to the fourth power of a lights wavelength and that the shorter a lights wavelength, the more than incisively visible it is.Hence, as GBIV light (400nm to 500nm) have shorter wavelengths comp ard to ROY light (570nm to 700nm), GBIV light is more intensely visible to us.Due to the shorter wavelengths of GBIV light, they in addition have higher chances of encountering atmospheric molecules than the ROY light and getting scattered by it, as shown in the figure below.Although scattered light continues travelling in a random direction, a greater proportion of GBIV than ROY light reaches our eyes because GBIV wavelengths are shorter and more easily scattered. However, as our eyes detect the blue component in GBIV light that has been scattered the easiest (Leong, 2006), we see the mid-day huckster as blue in colour.1.2 Sunrise and sunsetAt morn and sunset, sunlight has to travel through a longer distance through the atmosphere than at mid-day before it is visible. As a result, sunlight encounters more atmospheric molecules during these times which results in more scattering of light. Hence, with this increased distance, regular the higher wavelengths (ROY) are scattered, and this work the set up reddish-orange.However, it is only at the horizons that the sky looks reddish-orange because the multiple rays of sunlight travel through different distances in the atmosphere due to refraction.As seen in the figure above, sunlight ray A penetrates the atmosphere very minimally and experiences very little refraction. As a result, it travels the shortest distance through the atmosphere and gets very little scattering. Thus, only the violet and indigo wavelengths are scattered. cheerfulness ray B enters more deeply into atmosphere than A and gets refracted more. Hence, it has to travel a longer distance through the atmosphere which scatters most of BIV wavelengths. Before it leaves the atmosphere, however, the remaining wavelengths are scattered which results in a portion of the sky looking yellow.Sunlight ray C penetrates deepest into the atmosphere and experiences the most refraction. As it travels the longest distance of the three rays, its higher red and orange wavelength s are overly scattered. This gives a portion of the sky its reddish-orange colour.Hence, this is why only the horizons are coloured reddish-orange while the rest of the sky remains blue during sunrise and sunset.1.3 Night-timeSunlight reflected off the moon undergoes Rayleigh scattering as well, although its effects are not as visible. As mentioned earlier, the intensity of the light source (I0) and the distance of the light source from the particle (R) also affects the how intensely visible the scattered light is.Although the Moon is more than closer to the Earth than the Sun, the light we get from it is much less intense than that from the Sun, and so reducing the intensity of scattered light.2. Mie scatteringAside from Rayleigh scattering, another type of scattering, in the form of Mie scattering, exists. This occurs when sunlight encounters molecules which are comparable in surface to its composite wavelengths. As a result, all visible wavelengths in sunlight are almost scat tered equally.2.1 CloudsOn a daily basis, Mie scattering is observable when we look at clouds. As water vapour in clouds tend to be 20 micrometres in diameter (University of Illinois, 2010), all visible wavelengths in sunlight are scattered by clouds equally, thus giving clouds their pureness appearance.Bigger clouds or rain clouds usually look darker because they are bigger and contain more water vapour molecules. Thus, as these clouds scatter most of the sunlight that pass through them, very little sunlight exits through the bottoms of these clouds. to a fault appearing white or grey, clouds can also appear reddish at sunrise or sunset. This is attributed to both Mie and Rayleigh scattering.At these times, sunlight travels a longer distance through the atmosphere and most of the shorter GBIV wavelengths are scattered just as sunlight enters the atmosphere. Hence, the further sunlight travels through the atmosphere, the greater the proportion of longer ROY wavelengths that remai n. Thus, it is mainly the ROY wavelengths that are able to reach and get scattered by the clouds and this causes clouds to appear reddish.Mie scattering is also observable on nights when moonlight is scattered by clouds in the sky and results in clouds appearing white.3. Pollution3.1 Light pollutionOn certain late nights, it is also possible to observe red clouds in the sky plane when there is no source of light in the atmosphere. This phenomenon is a result of denser clouds scattering light from the metropolis back down onto Earth. As Singapore is by and large lit up by yellow or orange light from streetlights, a greater proportion of longer wavelengths are scattered by the clouds thus making the clouds look red.However, if the clouds do not contain enough water vapour molecules and hence, are not large or dense enough, less city light is scattered. Thus, the redness visible in these clouds is not as pronounced and this is why red clouds are not always visible at night. anyhow Mie scattering, city light also undergoes Rayleigh scattering at night, although its effect is not as obvious as scattered sunlight. While city light is broadly emitted in longer wavelengths and do not experience much Rayleigh scattering, its shorter wavelengths are greatly scattered by atmospheric particles as it travels through the atmosphere.Consequently, the shorter wavelengths of light scattered through Rayleigh scattering, together with the light scattered by clouds, increase the ambient smartness of Singapores night sky and inadvertently blocks out dim interstellar light that would otherwise be visible. Thus, this is why it is quite difficult to see deep-space objects in most of Singapore with the naked-eye.3.2 HazeWhen dust and smoke particles from murk are present in the atmosphere, it becomes denser and causes light to undergo both Rayleigh and Mie scattering. This is because these particles typically range from 2.5 micrometres to 10 micrometres in diameter (Association of South East Asian Nations, 2014) and hence, are larger than visible light wavelengths.Thus, when the PSI is in the moderate range or worse, it is possible to see that all light appears muted and dull.At night, haze also causes Mie scattering of city light not only in clouds but throughout the sky. This results in significant portions night sky taking on a reddish glow.4. Personal reflectionsThrough this bemuse, I now have a better understanding of why the sky and clouds appear to change in colour over time as well as why it is relatively difficult to observe the cosmos in a brightly lit city at night. The latter was evident when our group spent a day at Changi beach to photograph the colours changes in the sky. Just as we were leaving the seaside after twilight, we noticed that were more stars were visible in the night sky than what can normally be seen in the brighter urban areas of Singapore. Unfortunately, we did not have a good enough camera that was able to photograph the star s in the night sky then.One of the interesting things I have learnt from this project is also why the sky looks red on hazy nights, even when there is no light from the Sun or Moon. This is primarily due to light pollution that is scattered back down to Earth by smoke and dust particles in the atmosphere. Similarly, denser clouds at night appear red due to light pollution being scattered back down by water vapour within them.It is also because of light pollution that the adage red sky at night, sailors delight no longer holds true all the time, especially in cities that are brightly lit up at night. This is because the red sky at night might not be due to a high-pressure system in the west bringing calm down weather, but due to manmade causes mentioned above (Teitel, 2012).This manmade phenomenon also has an implication on our circadian rhythms that act as our natural body clocks. Brighter skies at night could confuse our brains into persuasion that it is daytime (and not time to s leep), thus disrupting our sleeping patterns and leaving us feeling jet-lagged. In the near future, the night sky might become even brighter as gas-discharge lights are replaced by light-emitting diode (LED) lights, which emit a greater concentration of shorter wavelengths (Teitel, 2012). Our circadian rhythms might be even more negatively bear on by this increase in blue light, as they are especially sensitive to shorter wavelengths. A potential solution to mitigate this problem is to use stiff white lights that emit minimal blue wavelengths.Additionally, understanding how light is affected by minute particles has also allowed humankind to apply these principles in real-life, such as in optical fibres.In optical fibres, light is used to transmit signals through cables made of transparent silica fibres. Since the silica molecules in these fibres are much smaller than the wavelength of visible light (Yao, 2010), Rayleigh scattering occurs within these cables and causes light to be scattered in a random direction instead of travelling through the cable. This is detrimental to signal strength.Hence, as Rayleigh scattering is extremely dependant on a lights wavelength and short wavelengths of light scatter the most, optical fibres usually transmit signals in longer light wavelengths to minimise scattering.5. ConclusionOne of the most observable phenomena by the naked eye, the sky is often photographed at sunrise or sunset for its picturesque clouds. Besides just appreciating the beauty of these occurrences, I now understand the science behind them as well. Additionally, I also now know why the sky or clouds are red on certain nights even when there is no light from the Sun or the Moon.Besides the theoretical principles and atmospheric observations, scattering of light also has practical applications in real-life. In times before satellite imaging existed to forecast weather, people referred to colours in the night sky to determine if there would be good weather the next day. Today, knowledge of light scattering is also useful in deciding what wavelengths of light should be used when transmitting signals through optical fibres.In addition, understanding Rayleigh and Mie scattering also allows us to minimise to impact that bright skies might have on our natural body clocks.Thus, the study of light scattering not only allows us to better appreciate our atmosphere, but also apply these concepts in real-life.(1949 words excluding citations)- End -6. BibliographyAssociation of South East Asian Nations. (2014). Information on Fire and Haze. Retrieved 07 March, 2014, from ASEAN Haze Action Online http//haze.asean.org/?page_id=249Leong, J. (2006). Number of colourize Distinguishable by the Human Eye. (G. Elert, Editor) Retrieved 26 March, 2014, from The Physics Factbook http//hypertextbook.com/facts/2006/JenniferLeong.shtmlNave, C. R. (2012). Blue Sky. Retrieved 17 March, 2014, from hyperphysics http//hyperphysics.phy-astr.gsu.edu/hbase/atmos/blus ky.htmlc4Seinfeld, J. H., Pandis , S. N. (2006). Atmospheric Chemistry and Physics From Air Pollution to Climate Change, 2nd Edition. raw(a) Jersey John Wiley and Sons.Siegel, E. (08 September, 2009). Red sky at night but why? Retrieved 05 March, 2014, from scienceblogs.com http//scienceblogs.com/startswithabang/2009/09/08/red-sky-at-night-but-why/Teitel, A. S. (23 August, 2012). Why Is the Night Sky Turning Red? Retrieved 28 February, 2014, from discovermagazine.com http//blogs.discovermagazine.com/crux of the matter/2012/08/23/why-is-the-night-sky-turning-red/University of Illinois. (2010). Scattering of Light. Retrieved 29 March, 2014, from WW2010 http//ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/mch/sct.rxmlYao, C. (20 September, 2010). Optical Fiber Loss and Attenuation. Retrieved 17 March, 2014, from Fibre Optics For Sale Co. http//www.fiberoptics4sale.com/wordpress/optical-fiber-loss-and-attenuation/GEK1520/PC1322 Understanding the UniversePage 1 of 13

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