(Note: Astronomers use mostly c.g.s) for frequency, a wide variety used     2n = c frequency defines l     e = hn frequency defines energy frequency 1 Hz cycles per second, only frequency used                                     MHz, GHz (radio)             but e ~ n and ~ n Some other interchangeable units Energy: 1 eV                     1eV -> n1eV = = 2.42 x 1014 Hz (OPTICAL) 1keV            1000eV -> 2.42 x 1017 Hz 1cm-1             = 3 x 1010 cm s-1 Wavelength 1Å                   1Å cm 1Å =>12.398 keV                                         optical, UV, X-ray 1mm                  1mm = 10-4 cm ,   IR 1mm                  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 second   dW steradians n of these go into dW counted n photons per dA per dn per dW per sec or n photons   This unit is called "photon intensity" Convert to "specific intensity" by multiplying by hn I 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