Optical measurements and band structure calculations are reported on 3D Dirac materials. The electronic properties associated with the Dirac cone are identified in the reflectivity spectra of Cd$_3$As$_2$ and Na$_3$Bi single crystals. In Na$_3$Bi, the plasma edge is found to be strongly temperature dependent due to thermally excited free carriers in the Dirac cone. The thermal behavior provides an estimate of the Fermi level $E_F=25$ meV and the z-axis Fermi velocity $v_z = 0.3 text{ eV} AA$ associated with the heavy bismuth Dirac band. At high energies above the $Gamma$-point Lifshitz gap energy, a frequency and temperature independent $epsilon_2$ indicative of Dirac cone interband transitions translates into an ab-plane Fermi velocity of $3 text{ eV} AA$. The observed number of IR phonons rules out the $text{P}6_3text{/mmc}$ space group symmetry but is consistent with the $text{P}bar{3}text{c}1$ candidate symmetry. A plasmaron excitation is discovered near the plasmon energy that persists over a broad range of temperature. The optical signature of the large joint density of states arising from saddle points at $Gamma$ is strongly suppressed in Na$_3$Bi consistent with band structure calculations that show the dipole transition matrix elements to be weak due to the very small s-orbital character of the Dirac bands. In Cd$_3$As$_2$, a distinctive peak in reflectivity due to the logarithmic divergence in $epsilon_1$ expected at the onset of Dirac cone interband transitions is identified. The center frequency of the peak shifts with temperature quantitatively consistent with a linear dispersion and a carrier density of $n=1.3times10^{17}text{ cm}^{-3}$. The peak width gives a measure of the Fermi velocity anisotropy of $10%$, indicating a nearly spherical Fermi surface. The lineshape gives an upper bound estimate of 7 meV for the potential fluctuation energy scale.
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