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( Note: Astronomers use mostly c.g.s)for frequency, a wide variety used e = hn frequency defines energy frequency 1 Hz cycles per second, only frequency used MHz, GHz (radio) but e ~ n and ~ nSome other interchangeable units Energy: 1 eV 1eV -> n1eV = = 2.42 x 1014 Hz (OPTICAL)1keV 1000eV -> 2.42 x 1017 Hz 1 cm-1 = 3 x 1010 cm s-1Wavelength 1Å
1Å cm 1Å =>12.398 keV 1 mm 1mm = 10-4 cm , IR1mm 1mm = 0.1 cm, Microwaves 1cm Radio Objects emit photons. Look at a small area dA, emits N photons of Frequency g <->n+dA per seconddW steradians n of these go into dW counted n photons per dA per dn per dW per sec This unit is called "photon intensity" Convert to "specific intensity" by multiplying by h nI has units of ergs cm-2 s-1 Hz -1 ster -1
"Flux density" is obtained when angle dependence is removed, at dA count all photons emited 1 Jansky = 1 Jy = 10-23 erg cm-2 s-1 Hz-1 Flux = F = -> erg cm-2 s-1 emitted Brightness = -> erg cm-2 s-1 ster -1 |