Rayleigh scattering

1
Rayleigh scattering

• Light scattering in the atmosphere

2
Contents

1. What is Rayleigh Scattering?
2. Light scattering in the atmosphere
3. Why is heaven blue?
4. Why is rising and setting sun redish?
5. Polarizsation
6. What else?

3
1. What is Rayleigh scattering?

• Named after John William Strutt,
• 3rd Lord of Rayleigh, 1871
• Definition
• elastic scattering of electromagnetic radiation
  (light), by particles much smaller than the
  wavelength of the radiation

4
1. What is Rayleigh scattering?

• The intensity I of light scattered by a single
  small particle from a beam of unpolarized light
  of wavelength ? and intensity I0 is given by
• R - distance to particle,
• ? - scattering angle,
• n - refractive index of particle,
• d - diameter of particle.

5
1. What is Rayleigh scattering?

• Integrating over the sphere surrounding the
  particle gives the Rayleigh scattering cross
  section ss
• The intensity of Rayleigh scattered light varies
  inversely with the fourth power of the wavelength
• Violet light is scattered stronger than red light

6
2. Light scattering in the atmosphere

• Sky is blue (or white if there are clouds)
• (exept in Bremen, where it is gray most of the
  time)
• reason
• scattering of
• sunlight by molecules
• or microparticles in
• the atmosphere

7
3. Why is heaven blue?
Source http//www.type.de/download/screens/bluesk
y_comp_04.jpg
8
3. Why is heaven blue?

• Intensity distribution of sunlight (? max455nm)
• Wavelength dependency of ss anti-proportional to
  ?4
• Spectral distibution of our eyes sensitivity
  ?(?)

9
3. Why is heaven blue?
...it isnt!! It is violet!

• physiological explanation
• our eyes are less sensitive to violet light
  (400 nm) than to blue light.
• Result the eye sees just the blue sky

http//en.wikipedia.org/wiki/Rayleigh_scattering
10
5. Why is rising and setting sun redish?

• Sunrise on Rügen
• from http//svenvogel.sv.funpic.de/Bilder/ruegen
  /Sonnenaufgang20Kap20Arkona09.JPG

11
5. Why is rising and setting sun redish?

• light passes through a greater thickness of the
  atmosphere
• multiple scatterings of blue light
• but relatively little scattering of red light
• red-hued sky in the direction towards the sun.

12
Polarization

• Unpolarized Sunlight induces dipoles oscillating
  perpendicular to direction of the baem k
• Intensity of scattering
• ISI0 sin2 ? I0 cos2 a
• Polarization dependent of visual angle
• P(1-cos2 a)/(1cos2 a)

13
6. What else? Mie scattering

• The general theory concerning scattering in the
  atmosphere is Mie theory (Gustav Adolf Ludwig
  Mie, 1868 1957)
• arbitrary size parameter
• in case of x ltlt 1 Mie theory reduces to
• Rayleigh approximation

14
6. What else? Mie scattering
15
List of literature

• Demtröder, Experimentalphysik 2, 2. Auflage,
  Springer
• Meschede, Gerthsen Physik, 21. Auflage, Springer
• Tipler, Physik, 1. Auflage, Spektrum Akademischer
  Verlag

16
End

• Thank you for your attention!
• Any questions?