Colours%20In%20The%20Sky

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Colours In The Sky

• By
• Harsh Gupta
• 11B

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Index

• Key Knowledge
• Types of waves
• Wavelength
• Frequency
• Amplitude
• Refraction
• Colour Despersion
• Colour Components
• Rayleigh scattering
• coefficient

• Why the sky is blue?
• Why isnt the sky violet?
• Why the sun is seen yellow?
• Changes during sunset and sunrise?
• Why is the sky black at night?
• Why do we see stars as a small dots?

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Types Of Waves

• There are two main types of waves
• Longitudinal wave
• Wave where the particles move right and
  left along the wave.
• Transverse Waves
• Wave where the particles move up and down
  along the wave.

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What is Light?

• Light is a transverse wave which always has a
  trough and a crest. The distance between two
  adjacent troughs or crests is the wavelength of
  the wave.

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Wave Length

• The wavelength is the distance between repeating
  units of a wave pattern. It is represented by the
  greek letter lambda (?).
• In a wave, the wavelength is the distance between
  peaks (Crests)

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Frequency

• Frequency is the number of waves made per second.
  This is usually measured in hertz (Hz).
• Here the frequency is 3Hz

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Amplitude
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Refraction

• It is the bending of light when it passes from
  one medium to another medium . When the light
  enters a denser medium it slows down and when the
  light enters a less denser medium it speeds up.

Incident ray - ray of light before it strikes the
block. Refracted ray - ray of light after it has
passed into the medium. Angle of incidence -
angle between the normal and the incident
ray. Angle of refraction - angle between the
normal and the refracted ray. Emergent ray - ray
that comes out on the opposite side of the block.
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Colour Dispersion

• Dispersion is refraction of white light into its
  component colours when it passes through a prism
  because the different speed of the colours inside
  the prism.
• Each colour is bent or refracted by a different
  amount.
• The colour pattern that is
• seen is called the spectrum
• of white light.

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Colour components

• A Prism can be used to refract white light
  into its component colours. This shows that white
  light is a mixture of the projected colours.

VIOLET INDIGO BLUE GREEN YELLOW ORANGE RED
Short Wave Length
Long Wave Length
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Why is Sky Blue?

• Optical Phenomenon.
• It is an event that can be observed which results
  from the interaction of light and matter.
• The blue in the sky we see is scattered blue
  light.

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Why is Sky Blue?

• The blue colour of the sky is due to Rayleigh
  scattering. As light moves through the atmosphere
• Most of the longer wavelengths pass straight
  through.
• Shorter wavelength light is scattered by the gas
  molecules.
• The scattered blue light is then
• radiated in different directions. It
• gets scattered all around the sky.

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Horizon

• To reach you, the scattered blue light
• must pass through more air. Some of
• it gets scattered away again in other
• directions. Less blue light reaches your
• eyes. The colour of the sky near the
• horizon appears paler or white.

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Rayleigh Scattering

• Rayleigh scattering (named after Lord Rayleigh
  or John William Strutt ) is the scattering of
  light by particles smaller than the wavelength of
  the light. It occurs when light travels in
  transparent solids and liquids, but is most
  prominently seen in gases.

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Rayleigh Law

• The amount of Rayleigh scattering that occurs
  to a beam of light, is dependent upon the size of
  the particles and the wavelength of the light.
  Considering this fact, he made the scattering
  coefficient.

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Rayleigh scattering coefficient

• The Rayleigh scattering coefficient ks is
• where n is the number of scatters of diameter
  d m is the index of refraction and ? is the
  wavelength of the radiation.

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Summary

• So, in the atmosphere, blue photons get scattered
  across the sky to a greater extent than photons
  of a longer wavelength, and so one sees blue
  light coming from all regions of the sky.

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What about Violet?

• If shorter wavelengths are scattered most
  strongly, then why does not the sky appear
  violet, which has the shortest visible
  wavelength. 
• There is less violet in the light emitted from
  the sun as the light is not constant at all
  wavelengths, and additionally is absorbed by the
  high atmosphere.
• Our eyes are also less sensitive to violet.

Response curves for the three cones
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Sunset and Sunrise

• The bending of sunlight causes the sun to be
  seen after it has dropped below the horizon, both
  sunrise and sunset are daily optical illusions.

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Change in Colour

• As the sun begins to set, the light must
  travel further through the atmosphere before it
  reaches the surface. More of the light is
  reflected and scattered. As less reaches you
  directly, the sun appears less bright.

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Yellow Sun

• On Earth, the sun appears yellow. Some of the
  shorter wavelength light (the blues and violets)
  are removed from the direct rays of the sun by
  scattering. The remaining colours together appear
  yellow.

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White Sun

• The sun looks white in space because there is no
  atmosphere to scatter the sun's light. This is
  also partly why the space looks dark and black.

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Amazing Fact

• Why does the sky appear black at night as there
  are many stars all around Earth to give light to
  and make the night sky bright?

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Simple Answer

• The night sky is black because the stars are so
  far away that by the time it reaches us, the
  luminosity of the star is gone. It is supported
  by the Inverse Square Law.

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Inverse Square Law
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Complicated Answer

• The night sky is dark because of the Olberss
  Paradox. In simple language, the fact that the
  universe had a finite age and is expanding is the
  principle explanation of Olberss Paradox

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Olberss Paradox

• After a long calculations it says The universe
  must be at least 6.6 X 1015 years old in order to
  make the night sky as bright as the surface of
  the sun
• According to studies, the universe is only about
  8 12 billion years old. So we wont be holding
  our breath waiting for the night sky to be as
  bright as the daylight.

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Why Stars as dots?

• The smaller the ? , the smaller we see the object
  no matter what its size is.
• If you take the object of the same size further
  far, the object would seem smaller.

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Reference

• http//www.learn.co.uk
• http//en.wikipedia.org/wiki
• http//www.why-is-the-sky-blue.tv/
• http//www.photolib.noaa.gov/searchttp
• http//weathersavvy.com/Q-BlueSky.html
• http//webexhibits.org/causesofcolor/14.html
• http//www.sky-watch.com/articles/skyblue.html
• http//science.howstuffworks.com/question39.htm
• http//www.sciencemadesimple.com/sky_blue.html
• http//ist-socrates.berkeley.edu/eps2/wisc/oLect7
  .html
• http//csep10.phys.utk.edu/astr162/lect/light/wave
  s.html
• http//www.gsfc.nasa.gov/scienceques2002/20030328.
  htm
• http//www.why-is-the-sky-blue.org/why-is-the-sky-
  blue.html
• http//www.newton.dep.anl.gov/askasci/phy00/phy007
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• http//www.glenbrook.k12.il.us/gbssci/phys/mmedia/
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• http//www.kettering.edu/drussell/Demos/waves/wav
  emotion.html
• http//eosweb.larc.nasa.gov/EDDOCS/Wavelengths_for
  _Colors.html
• http//www.personal.psu.edu/users/e/l/elw165/class
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• http//math.ucr.edu/home/baez/physics/General/Blue
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