7 Radiation from Space

Electromagnetic Radiation

c=l n "law of waves"

E=hn =hc/l "law of photons"

l1/l2=(d2/d1)2 "inverse square law" d=distance from light source

l=energy received by detector

Electromagnetic spectrum

V.L.F. Radio Microwave I.R. Visible U.V. E.U.V. X g

10 MHz GHz 1012 1014 1015 1016 1018 Hz

Wave particle duality:

propagation is wave - like

interaction is particle - like

Detection of Electromagnetic Radiation

Laws of Geometrical Optics

a) Reflection and Refraction

  1. The normal, the incident, transmitted and reflected rays are all coplanar.
  2. Angle of incidence (j ) =angle of reflection (j )
  3. sin j =n’ sin q (Snell’s law)
  4. n=c/v

b)Dispersion

Prism spectrograph

Atmospheric Dispersion

Rainbow

Halos

Spectroscopy

To Newton the sun’s spectrum appeared as a continuous spectrum.

In 1802 Wollaston observed dark lines in the spectrum but attributed these to "Color boundaries".

In 1814 Fraunhofer carried out an in depth study of these lines now called "Fraunhofer lines"

Subsequently, it was found that these lines could be produced in the laboratory.

Kirchoff’s Laws

  1. Luminous solid/liquid: emits light of all wavelengths (continuum).
  2. Rare luminous gas: emits bright lines (emission lines) and a faint continuum.
  3. White light through cool gas: emits a continuous spectrum with dark lines (absorption lines).

Examples:

1. The Fraunhofer spectrum

PICTURE

2. Emission nebulae

PICTURE

3. Interstellar absorption

4. Planetary atmosphere

5. Synchrotron

6. Cerenkov

7. Interstellar maser

a) Value of Spectroscopy

Each molecule/atom/ion has a unique spectrum.

Chemical analysis:

Qualitative - which lines are there

Quantitative - how fat the lines are

Plasma diagnostics:

Temperature

Gas motions

Density

etc.

Spectral Analysis

b) Doppler Effect, 1842.

To the first order, the radial component of the velocity of an object radiating electromagnetic radiation (EMR) will cause a shift in the wavelength of the emitted spectrum,

D l =(v/c)l =b l D l =l ‘-l

The exact formulae is:

l ‘=l (1-b 2).5/(1-b cosf )

D l =l ((1-b 2).5/(1-b cosf )-1)

If f =0 D l =l ((1+b ).5/(1-b ).5-1)

The Doppler effect tells us how fast stars approach/recede from us.

7.4 Radiation Laws

a) Radiators:

I) perfect radiator = blackbody radiator

A hypothetical cavity that absorbs all incoming radiation and reflects and transmits none. Reaches an equilibrium temp. and then reradiates energy as soon as it absorbs it.

II) grey radiator = greybody

Kirchoff’s Law:

Emissivity=1-Reflectivity

e =1-n

A perfect mirror (n=1, e =0) will radiate no heat, no matter how hot it is!

III) Normal radiator

e n =1-nn PICTURE

"Greenhouse effect": make a body transparent to visible and near IR, and opaque in the far IR and microwave.

"Reverse greenhouse effect": Telescope domes.

b) Planck’s Radiation Law (1900)

PICTURE Bl =(2hc2)/(l 5(ehc/l kT-1))

 

c) Wien’s law

The wavelength of peak emission is inversely proportional to the temperature. l maxT=const. [dB/dl =0]

D l =const. l maxT=2.8979 mm deg

passband l Tmax=3.67 mm deg

D n = const l maxT=5.100 mm deg [dB/dn =0]

 

d) Stefan- Boltzman law

E(T)=s T4 =ò Bl dl

s =5.67032x10-8 Wm2K-4

The total amount of E-M radiation emitted over all wavelengths is extremely sensitive to temperature

 

Absorption and Emission of Light by Atoms and Molecules

The Hydrogen Spectrum PICTURE

Balmer, a Swiss schoolteacher and numerologist, was challenged by a friend to find a formula relating 6563, 4861, 4340 and 4101 (Ha , Hb , Hg and Hd ). He found l =3645.6(n2/n2-4)

Balmer announced his discovery in 1885 at the age of 60. It was his first scientific contribution.

Rydberg showed for the alkali spectra:

1/l =R(1/m2-1/n2) m>1, n>m.

PICTURE

Absorption and Emission of EMR

Types:

1. Bound-Bound (Line)

2.Bound-Free (Continuum, recombination, photoionization)

3.Free-Free (Continuum, Bremsstrahlung)

4.<Dielectric recombination - Auger effect>

5.Scattering

 

Phenomena:

1.Stimulated absorption (absorption)

2.Stimulated emission

3.Spontaneous emission

 

Rules:

1.Parity must change

PICTURE PICTURE

2.etc.

3.etc.

4.etc.

PICTURE

Cosmic Rays

These are charged particles (mainly protons) which strike the molecules in the earth’s upper atmosphere, at near light speed, producing large amounts of secondary subatomic particles.

The primary cosmic rays have energies 108 - 1020 eV. Particles with energies over 1015 can produce a large number of secondary cosmic rays over a large area - Auger shower.

Their presence varies with latitude because of the earth’s magnetic field.