Why is the sky blue

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					Why is the sky blue?

  Notes: April 26th 2011
      Something to ponder…?
• When you look at the sky at night it is
  black, with the stars and the moon forming
  points of light on that black background. So
  why is it that, during the day, the sky does
  not remain black with the sun acting as
  another point of light? Why does the
  daytime sky turn a bright blue and the stars
              Light scattering
• 1. Sun is an extremely
  bright source of light.

• 2. We have an
  atmosphere. Small
  atoms of oxygen and
  nitrogen in the
  atmosphere effect

• Think of ringing bells!
         Rayleigh scattering
• Physical phenomenon that causes light to
  scatter when it passes through particles.

• Tiny particles bend high frequency light.
  (oxygen, nitrogen)- blue mountain skies

• Large particles bend low frequency light.
  (methane, sulfur)- sunsets in LA!
     Blue sky… why not violet?
• This scattering of the higher frequencies of light
  illuminates the skies with light on the BIV end of
  the visible spectrum.
• Violet is scattered most easily, so why isn’t the
  sky violet?
• Our eyes are more sensitive to light
  with blue frequencies.
• Three primary color cones in our eye:
  Blue, green, and red.
          Why is the sun yellow?
• The lower frequencies
    of sunlight (ROY)
  tend to reach our eyes
     unscattered as we
    look directly at the
  midday sun. Although
  sunlight is a combo of
   all colors, yellow is
       most intense.
           Appearance of sun
• Appearance of the sun changes with the time of
• As the path which the light takes through the
  atmosphere increases ROYGBIV encounters
  more atmospheric particles, and scatters more
• During sunset hours, the light passing through
  our atmosphere to our eyes tends to be
  concentrated with red and orange frequencies.
  This is why sunsets have a reddish-orange hue.
         What effects a sunset?
• The effect of a red sunset becomes more
  pronounced if the atmosphere contains more
  and more sulfur aerosols (industrial pollutant)
Natures Beauty
Endless beauty
                 Blue haze
• Some mountain ranges are famous for their blue
  hazes which result from aerosols from the
  vegetation reacting with ozone to form small
  particles (200 nm) which scatter blue light.
               Tyndall Effect
• John Tyndall (1859)
  discovered that when
  light passes through
  clear fluid holding
  small particles in
  suspension, the shorter
  blue wavelengths are
  scattered more
  strongly than red.
          Critical Opalescence
• People with blue eyes,
  really don’t have blue
  eyes. The fluid in our
  eyes scatters or
  reflects the blue light
  from our eyes.
• In the cells of a blue-
  jay feather.
• Blue gem stones.
         Why is Mars sky red?
• Images from Viking Mars landers (1977), and
  Pathfinder (1997) show a red sky from the
  Martian surface. This is due to a red iron rich
  dusts (magnetite) thrown up in dust storms,
  which absorbs blue wavelengths and reflects red.
       Why are clouds white?
• Clouds appear
  white because they
  consist of water
  droplets that are
  many sizes and
  scatter all
• (Mie scattering)
 Why is sea water greenish blue?
• Water absorbs
  infrared waves
  (heats water)
• Absorption of red
  light by water.
• What is left is the
  complement of red,
  which is cyan.
  (Greenish blue)
       Why are sea creatures red?
• In deep sea water,
  since there is no red
  light coming in to be
  reflected, the lobsters
  and other sea creatures
  look black, and
  through evolution
  have avoided being
  eaten by their
               In Summary
• The sky is blue because blue from sunlight
  is scattered in all directions by molecules in
  the atmosphere.
• Sunsets are red because all the other color
  frequencies are filtered out.
• Water looks greenish blue because water
  absorbs red light.
• The color of things depend on what colors
  are reflected or absorbed by molecules.

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