How Blue the Sky?

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How Blue the Sky?

• Physics Teachers Day
• 6 July 2007
• Andrew Hanson

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James Dewar

• None of you asses Can liquefy gasses!
• Invented flask for holding liquefied gases
• There is a lunar crater named after him
• The sky is blue because liquefied oxygen is
  blue.

• Sir James Dewar, 20 Sept 1842 27 Mar 1923,
  Scottish chemist and physicist
• Clerihew Sir James Dewar Is better than you
  are.

True
True
Wrong!
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A physics presentation brought to you in
association with

• What is Colour.The Human Visual System
• The Suns Colour.Plancks Formula
• Odd absorptions.Atomic Molecules
• Blue more than red..Raleigh Scattering
• Travel Arrangements...Expedia.com
• Measurements..National Physical Laboratory.

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What is colour?

• Colour is human coding of light.

What on earth is colour?
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What is Colour?

• What is colour? Its to do with the way we
  perceive light

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Afterimages
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Seeing Light 1
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Seeing Light 2 (in colour)
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What is colour?
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What is colour?
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What is colour?
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What is colour?
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What makes things colourful?(The Physics
Chemistry of ColorThe 15 Causes of Color Kurt
Nassau)
Vibrations, simple excitations, rotations incandescence Flames
Vibrations, simple excitations, rotations gas excitations neon tube, Aurora
Vibrations, simple excitations, rotations rotations blue ice and water
Ligand-field-effect colours transition-metal compounds turquoise, chrome-oxide green
Ligand-field-effect colours impurities ruby, emerald
Molecular orbitals organic compounds indigo, chlorophyll
Molecular orbitals charge-transfer compounds blue sapphire, lapis lazuli
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What makes things colourful?(Things that cause
things to happen differentlyat different optical
wavelengths)
Energy bands metals and alloys gold, brass
Energy bands semiconductors cadmium yellow, vermilion
Energy bands doped semiconductors blue and yellow diamond
Energy bands colour centres amethyst, topaz
Geometrical and physical optics dispersive refraction rainbow, green flash
Geometrical and physical optics scattering blue sky, blue eyes, red sunset
Geometrical and physical optics interference soap bubbles, iridescent beetles
Geometrical and physical optics diffraction the corona aureole, opal
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What makes things colourful?

• Non-flat spectra.

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Dr. Pete Vukusic leads natural photonics
research at Exeter
http//newton.ex.ac.uk/research/emag/butterflies/
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So much for what colour is..

• Why is the sky the colour it is?

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So much for what colour is..

• Why is the sun the colour it is?

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Max Planck

• Max Karl Ernst Ludwig Planck, 23 April 1858 - 04
  October 1947
• Reality is ... just a very specific, narrow
  slice of that vast range of what our thoughts
  try to encompass. February 1923
• Was a good musician and had Albert Einstein
  round now and then to play music together
• Had a son who was executed for trying to
  assassinate Hitler
• Was asked by electric lamp companies to
  calculate how to get most light out of electric
  lamps..

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Why is the sky the colour it is?Part 1 The Sun
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Planckian radiators
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Why is the sky the colour it is?Part 2
Molecular absorption

• Matter in the sun and the earths atmospheres
  keenly absorb light at some wavelengths
• This tuned absorption relates to the types of
  bonds between either electrons and nucleus in
  atom atoms in molecules
• It is as though we pass wind (containing all
  frequencies) over organ pipes which only get
  excited at some pitches.

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Why is the sky the colour it is?Part 2
Molecular absorption
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Story so far

• 1 Sun (hot object)
• 2 Sun and Atmosphere (absorb some wavelengths)
• 3 Atmosphere scatters light.

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John Tyndall

• John Tyndall, 02 Aug 182004 Dec 1893 Irish
  Natural Philosopher
• "this aqueous vapour is a blanket more necessary
  to the vegetable life of England than clothing
  is to man. Remove for a single summer night the
  aqueous vapour from the air that overspreads
  this country, and you would assuredly destroy
  every plant capable of being destroyed by a
  freezing temperature.
• One of first scientists to suggest greenhouse
  gases
• Suggested that sky blueness due to light
  scattered by atmospheric dust
• Proposed scattering inversely proportional to the
  fourth power of the wavelength.

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Lord Raleigh,John Strutt, 3rd Baron Rayleigh

• John William Strutt, 3rd Baron Rayleigh English,
  12 Nov 1842 30 June 1919
• Rayleigh criterion, Rayleigh fading, Rayleigh
  number,Rayleigh quotient, Rayleigh scattering,
  Rayleigh waves,Rayleigh-Jeans law, Rayleigh
  distribution, Rayleigh-Taylor instability,
  Rayleigh (unit) (named after his son)
• Discovered Argon
• There are craters on the Moon and Mars named
  after him.

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Why is the sky the colour it is?Part 3
Scattering
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Why is the sky the colour it is?Part 3
Scattering

• Raleigh scattering simplified intensity
  proportional to 1/wavelength4)

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The unscattered light is red.
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How can you measure blue skies?

• Use a camera or spectroradiometer
• Camera captures R, G, B values
• Spectroradiometer captures spectral data for
  manipulation
• We can represent all colours on a chromaticity
  chart
• We can represent some colours using a colour
  temperature scale.

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The Challenge..

• Where on earth is the bluest sky?

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Anya Hohnbaum

• Anya Hohnbaum,Portuguese-German-Scottish
  1978-TV Production Coordinator/Tennis Coach
• There are no craters on the Moon or Mars named
  after her. yet
• Won Expedia.com Competition to findbluest sky on
  earth.

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1 x 64,000 mile air ticket

• 1752 Hours
• 72 Days
• 25 destinations
• 22 countries
• 57 flights

• 15 tonne of CO2

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Apparently Anya had quite an interesting time.
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Measuring sky spectrum
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Combine spectra with an eye model
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Plot results on a colour chart
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Colour coordinates of spectral sky results
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Photograph the sky
Photograph of the NPL test card
Photograph of the sky overhead
Photograph of the horizon

Photograph with the fisheye lens
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Analysis of the photographs
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Expedias Best Blue Sky is
Brazil

• The sky colour can be specified as
• x 0.2775 y 0.2842
• colour temperature 10,637 K (10,910 C).

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• How blue the sky?
• This talk charts two voyages of discovery. The
  first around physics tries to explain what colour
  is, how we measure it and answers that most
  common query why is the sky blue? The trip
  visits topics as diverse as (humane) animal
  experimentation, the colour of an object heated
  to infinity degrees Celsius, post box culture,
  greenhouse gases and optical-molecular
  scattering. The second voyage is more physical
  an international scientific expedition made in
  2006 to locate the worlds bluest sky. But what
  do we mean by bluest? How do we measure blueness?
  Most importantly, where is the bluest sky on
  earth?