1	The Nature of Light Lecture 6 Ay-1
2	Electromagnetic radiation emitted by astronomical objects
3	electromagnetic waves ® transfer of energy by a disturbance Young’s experiment ® wave nature l = wavelength = length between crests/valleys n = frequency = number of crests passing per unit time = cycles per sec (Hz) = 1/P P = period = time for wave to repeat itself nl = c (speed of light - celeritas) = 3 x 10¹⁰ cm/sec = 300, 000 km/s visible light: 4000 < l > 8000 Ångstrom (1Å = 10^-8 cm) For a star, let B= brightness = power/area = power/4pd² If L = total output of star, L = Bx4pd² and B = L/4pd²
4	Waves and charged particles charged particles (electrons, protons) and carry electric field, strength of field µ 1/d² collision or heating ® particle vibration ® change in field ® change in electrical forces on other particles change in electrical forces ® information about original particle ®information transmitted through a disturbance (change in electrical field) magnetic field associated with electrical field ®electromagnetic waves Electromagnetic waves from moving charged particles in astronomical objects propagate at speed of light, c (need not be visible)
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6	Two main processes for e-m radiation Thermal radiation constituent particles in constant random motion ® e-m radiation temperature @ amount of motion radiation emitted over range of frequencies Spectral line radiation discrete quantum mechanical line radiation -- only at n corresponding to quantized energy level differences
7	Blackbody Radiation Blackbody absorbs (& re-emits) all energy falling onto it characteristic of any opaque surface (e.g. stove burner depends only on T of surface (not shape or composition) Kirkoff’s Law : perfect absorber = perfect emitter = Blackbody Intensity of radiation versus frequency ® blackbody curve
8	Blackbody (Planck) curves at different temperatures
9	Radiation laws – apply to astronomical sources Wien’s law : l_peak= 0.289 / T(K) cm l_peak® color: hotter ® bluer (shorter l) cooler ® redder (longer l) Greater total energy (summed over all wavelengths) radiated by hotter objects ¯ Stefan’s Law : energy radiated/unit area of B-B surface/unit time µ (temp)⁴ F = sT⁴ Flux = 5.67x 10^-8T⁴ watt/m²/K⁴ Stefan’s constant = s
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11	e-m radiation ® stellar properties brightness ® energy output B = L/4pd² (if distance to object/star is known) color ® surface temperature (T) of star (blackbody) l_peak= 0.289 / T(K) how do we measure stellar distance?
12	Measuring stellar distances
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17	e-m radiation: spectral lines Quantum transitions energy levels within atomic and molecular systems quantized only discrete orbits/energies permitted when atom makes a transition between 2 states, gives up energy corresponding to the difference discrete amounts of energy only (photons) types of transition collisional excitation/deexcitation radiative absorption + spontaneous emission; stimulated emission Photon energy µ radiation frequency (color) DE = E_final - E_initial = hn (radiation) or ½mv² (collisional)
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21	Kirchhoff’s Laws Relate continuous spectra, emission line spectra, & absorption line spectra Luminous solid (dense gas) emits light of all wavelengths ® continuous spectrum of radiation Low density hot gas emits spectrum of bright emission lines ® composition of gas Low density cool gas absorbs wavelengths from continuous spectrum ® dark absorption lines on continuous spectrum (wavelengths same as emission lines from hot gas)
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