Light

Photons The photon is the gauge boson of the electromagnetic force. –Massless –Stable –Interacts with charged particles. Photon velocity depends on the medium. –c =  10 8 m/s –n = index of refraction The light year is a distance, 1 ly = 9.5  km.

Measuring Photons Photons can act as waves or particles. Wavelength ( ) and frequency ( ) are associated with waves. –Preferred for low energy photons Energy is associated with particles. –Preferred for high energy photons in units of eV hc = keV nm

Electromagnetic Radiation Traditional upper boundaries for types of EM radiation:  (m)  (Hz)E (eV) Radio waves 13   Microwaves1    Infrared0.75   Visible light0.4   Ultraviolet light1.2    10 2 X-rays1.4    10 5 Gamma rays (highest energy)

Sources of Photons Accelerated charges emit photons. –Continuous or discrete spectra may result Moving charge Emitted photon Photons can be reabsorbed as well.

Kirchhoff’s Radiation Radiated electromagnetic energy is the source of radiated thermal energy. –Depends on wavelength Objects can emit and absorb electromagnetic energy. –Emission coefficient  –Absorption coefficient  Expect a distribution I that depends on temperature.

A black object is perfectly absorbing. –Absorption coefficient is 1 The distribution is just due to emission. An isolated cavity with a narrow hole radiates like a perfectly black body at the same temperature (1859). Black Body

Blackbody Thermodynamics Assume the cavity has particles which interact with the wall. –Relativistic photon energy –Relate to energy density Apply the 2 nd law to the energy. –Stefan-Boltzmann law Real objects have a factor for emissivity .

Quantized Blackbody The power spectrum is defined by the power per unit area per unit wavelength. –Differential spectrum –W/m 3 or Wcm -1  m -1 The integral is the Stefan- Boltzmann law. For large E=h

low energy high energy frequency intensity Blackbody Radiation Heated gas radiates electromagnetic energy as blackbody radiation. The frequency spectrum power is a function of temperature. –W (,T) Earth surface: 300 K  20 ºC Sun surface: 5800 K  6100 ºC Sun interior: 1.57  10 7 K

Atoms and Light Atomic electron energy levels are a source of discrete photon energies. Change from a high to low energy state produces a photon. Atoms can also absorb a photon to excite an electron.

Hydrogen Hydrogen is the most common element. Emission series for hydrogen have defined names for inner n. –Lyman 1 –Balmer 2 –Paschen 3 n = 1 n = 2 n = 3

Discrete Spectrum Each atom has its own set of energy levels. Each atom generates photons at specific frequencies. The pattern of frequencies identifies the atom. helium neon

Absorption Lines Ionized gases at a star’s surface absorb specific frequencies of light. These appear as dark lines in a star’s spectrum. Since gases ionize at different temperatures, the appearance of lines indicate the temperature of the star.

Molecular Spectra Energy states in molecules contribute to stellar spectra. Internuclear distances are quantized in discrete states. –Vibrational energy Angular momentum for the molecule is quantized. –Rotational energy

Fluorescence Atoms and molecules can reemit absorbed energy. Fluorescence typically involves three steps. –Excitation to higher energy state. –Loss of energy through change in vibrational state –Emission of fluorescent photon s S1S1 S0S s s

X-Rays X-rays are associated with energetic transitions in atoms. Continuous spectra result from electron bombardment. Discrete spectra result from electron transitions with an atom. target electrons x-ray

Bremsstrahlung Acceleration of a charged particle is associated with a photon. –Bremsstrahlung means braking radiation –Electrons passing through matter –Continuous spectrum x-rays e  e Z

Photoelectric Effect A photon can eject an electron from an atom. –Photon is absorbed –Minimum energy needed for interaction. –Cross section decreases at high energy e  Z

Compton Effect Photons scattering from atomic electrons are described by the Compton effect. –Conservation of energy and momentum   e  ’’

Compton Energy The frequency shift is independent of energy. The energy of the photon depends on the angle. –Max at 180° Recoil angle for electron related to photon energy transfer –Small   cot large –Recoil near 90°

Gamma Rays Gamma rays are photons associated with nuclear or particle processes. –Energy range overlaps: soft gamma equals hard x-ray Nuclear gamma emissions are between isomers. –A and Z stay constant –Distinct energies for transitions

Nuclear Radiation Nuclear decay can leave a nucleus in an excited state. –Many possible states may be reached –Lifetime typically s Excess energy may be lost as a photon or electron. –Single gamma –Series of gamma emissions –Internal conversion beta MeV MeV 0 MeV 94.4%  5.5%  2.2% 3.3% 