blue sky
E.M. INTERACTION
WITH MATTER
1
SCATTERING of E.M. RADIATION
Type of scattering depends on the e.m. energy, ~! wrt the electron energy:
Ionization energy EI
Rest-mass energy me c2
2
CLASSICAL MODEL
E.M. wave interacting with an oscillating electron
Frequency of oscillation is given by Coulomb energy: !02 = kC /me
e.m. field adds a driving force at e.m. frequency !, F =
Radiated power is related to the electron acceleration, a
2
✓
◆2
!4
2
cE
(!02 ! 2 )2 0 cE02 = P/I0 , I0 =
The cross-section is then
8⇡
✓ 2 ◆2 ✓
◆2
2
8⇡
e
!
=
3 me c2
!02 ! 2
2e 2
1
P = a =
3
3c
3
2
e me c2
3
eE0 sin(!t)
RAYLEIGH SCATTERING
At very low energy ! ⌧ !0 cross-section becomes:
8⇡
=
3
✓
2
e me c2
◆2 ✓
!
!0
◆4
Very strong depence on the frequency/wavelength shorter wavelengths are scattered most longer wavelengths travel in (almost) straight lines
Rayleigh scattering experiment?
4
RAYLEIGH SCATTERING
Why is the sky blue?
The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air. However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.
Why is it paler close to the horizon?
As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.
Courtesy of Science Made Simple. Used with permission.
5
RAYLEIGH SCATTERING
Why is the sky red at sunset?
As the sun begins to set, the light must travel farther through the
WITH MATTER
1
SCATTERING of E.M. RADIATION
Type of scattering depends on the e.m. energy, ~! wrt the electron energy:
Ionization energy EI
Rest-mass energy me c2
2
CLASSICAL MODEL
E.M. wave interacting with an oscillating electron
Frequency of oscillation is given by Coulomb energy: !02 = kC /me
e.m. field adds a driving force at e.m. frequency !, F =
Radiated power is related to the electron acceleration, a
2
✓
◆2
!4
2
cE
(!02 ! 2 )2 0 cE02 = P/I0 , I0 =
The cross-section is then
8⇡
✓ 2 ◆2 ✓
◆2
2
8⇡
e
!
=
3 me c2
!02 ! 2
2e 2
1
P = a =
3
3c
3
2
e me c2
3
eE0 sin(!t)
RAYLEIGH SCATTERING
At very low energy ! ⌧ !0 cross-section becomes:
8⇡
=
3
✓
2
e me c2
◆2 ✓
!
!0
◆4
Very strong depence on the frequency/wavelength shorter wavelengths are scattered most longer wavelengths travel in (almost) straight lines
Rayleigh scattering experiment?
4
RAYLEIGH SCATTERING
Why is the sky blue?
The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air. However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.
Why is it paler close to the horizon?
As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.
Courtesy of Science Made Simple. Used with permission.
5
RAYLEIGH SCATTERING
Why is the sky red at sunset?
As the sun begins to set, the light must travel farther through the