Transcript Slide 1

Rayleigh scattering
Light scattering in the atmosphere
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Contents
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2.
3.
4.
5.
6.
What is Rayleigh Scattering?
Light scattering in the atmosphere
Why is heaven blue?
Why is rising and setting sun redish?
Polarizsation
What else?
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1. What is Rayleigh scattering?

Named after John William Strutt,
3rd Lord of Rayleigh, 1871
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Definition:
elastic scattering of electromagnetic radiation (light),
by particles much smaller than the wavelength of the
radiation
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1. What is Rayleigh scattering?

The intensity I of light scattered by a single small particle
from a beam of unpolarized light of wavelength λ and
intensity I0 is given by:
R - distance to particle,
θ - scattering angle,
n - refractive index of particle,
d - diameter of particle.
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1. What is Rayleigh scattering?

Integrating over the sphere surrounding the particle
gives the Rayleigh scattering cross section σs:

The intensity of Rayleigh scattered light varies inversely
with the fourth power of the wavelength
Violet light is scattered stronger than red light

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2. Light scattering in the atmosphere
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Sky is blue (or white if there are clouds)
(exept in Bremen, where it is gray most of the time)
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reason:
scattering of
sunlight by molecules
or microparticles in
the atmosphere
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3. Why is heaven blue?
Source: http://www.type.de/download/screens/bluesky_comp_04.jpg
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3. Why is heaven blue?
 Intensity distribution of sunlight (λ max=455nm)
 Wavelength dependency of σs anti-proportional to λ4
 Spectral distibution of our eyes‘ sensitivity η(λ)
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3. Why is heaven blue?
...it isn‘t!!
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
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http://en.wikipedia.org/wiki/Rayleigh_scattering
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5. Why is rising and setting sun
redish?
Sunrise on Rügen
from: http://svenvogel.sv.funpic.de/Bilder/ruegen/Sonnenaufgang%20Kap%20Arkona09.JPG
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5. Why is rising and setting sun
redish?
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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.
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Polarization
 Unpolarized Sunlight induces
dipoles oscillating
perpendicular to direction of the
baem k
 Intensity of scattering:
IS=I0 sin2 θ = I0 cos2 α
 Polarization dependent of
visual angle
P=(1-cos2 α)/(1+cos2 α)
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6. What else? Mie scattering
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The general theory concerning scattering in the
atmosphere is Mie theory (Gustav Adolf Ludwig Mie,
1868 – 1957)
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arbitrary size parameter:
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in case of x << 1 Mie theory reduces to
Rayleigh approximation
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6. What else? Mie scattering
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List of literature
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Demtröder, Experimentalphysik 2, 2. Auflage,
Springer
Meschede, Gerthsen Physik, 21. Auflage,
Springer
Tipler, Physik, 1. Auflage, Spektrum
Akademischer Verlag
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End
Thank you for your attention!
Any questions?
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