Problem 1 Why is the Sky blue?

Scattering efficiency ω4∞

So the higher the frequency the stronger the scattering

Blue light is much more strongly scattered than red

light

As the Sun sets only red light manages to get through

The blue is completely scattered away

650 600 550 500 450

Wavelength λ (nm)

Relative Scattering

25

Blue light is 4x more strongly scattered than

red light

20/5 = 4

0

5

10

15

20

Black Clouds towards the Galactic Centre

Blue light scattered strongly by interstellar gas and dust clouds

Problem

Determine the relative scattering efficiencies:

a) S (Hg) / S (Na)

b) S(NH3) / S (Na)

Na(D) 580 nm

Hg (blue) 405 nm

NH3 (radio line) 1 cm

Scattering by interstellar Gas and Dust3

2

1

Magnitude39.8

15.8

6.3

2.5

10,000 20,000 cm-1

3x1014 6x1014 Hz (cps)

Red Blue

650 600 550 500 450 λ (nm)

25

20

15

10

5

0

Relative cross-

sections for Rayleigh

scattering as a function

of wavelength

Scattering by interstellar Gas and Dust though 6000 ly of the ISM

3

2

1

Magnitude

39.8

15.8

6.3

2.5

10,000 20,000 cm-1

3x1014 6x1014 Hz

seen under mercury-vapour light, 1995.

Victoria Road in the District of Bradmore, Wolverhampton on the evening

of Wednesday 27th December 1995. At the time, this road was still lit with old fashion mercury-vapour

Harry Kroto 2004

Rayleigh scattering

http://www.ccpo.odu.edu/~lizsmith/SEES/ozone/class/Chap_4/index.htm

Scattering by interstellar Gas and Dust

Harry Kroto 2004

3

2

1

Magnitude

39.8

15.8

6.3

2.5

10,000 20,000 cm-1

3x1014 6x1014 Hz (cps)

Red Blue

Attenuation due to scattering by interstellar gas and dust clouds

Harry Kroto 2004

Harry Kroto 2004

Harry Kroto 2004

Rayleigh ScatteringRayleigh scattering refers to the scattering of light off of the molecules of the air, and can be extended to scattering from particles up to about a tenth of the wavelength of the light. It is Rayleigh scattering off the molecules of the air which gives us the blue sky. Lord Rayleigh calculated the scattered intensity from dipole scatterers much smaller than the wavelength to be: Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons is not changed. Scattering in which the scattered photons have either a higher or lower photon energy is called Raman scattering. Usually this kind of scattering involves exciting some vibrational mode of the molecules, giving a lower scattered photon energy, or scattering off an excited vibrational state of a molecule which adds its vibrational energy to the incident photon.

Mie ScatteringThe scattering from molecules and very tiny particles (< 1 /10 wavelength) is predominantly Rayleigh scattering. For particle sizes larger than a wavelength, Mie scattering predominates. This scattering produces a pattern like an antenna lobe, with a sharper and more intense forward lobe for larger particles. Mie scattering is not strongly wavelength dependent and produces the almost white glare around the sun when a lot of particulate material is present in the air. It also gives us the the white light from mist and fog. Greenler in his "Rainbows, Haloes and Glories" has some excellent color plates demonstrating Mie scattering and its dramatic absence in the particle-free air of the polar regions.