  Space-time (ST) wave packets are pulsed optical beams endowed with precise
spatio-temporal structure by virtue of which they exhibit unique and useful
characteristics, such as propagation invariance and tunable group velocity. We
study in detail here, and in two accompanying papers, the refraction of ST wave
packets at planar interfaces between non-dispersive, homogeneous, isotropic
dielectrics. We formulate a law of refraction that determines the change in the
ST wave-packet group velocity across such an interface as a consequence of a
newly identified optical refractive invariant that we call 'the spectral
curvature'. Because the spectral curvature vanishes in conventional optical
fields where the spatial and temporal degrees of freedom are separable, these
phenomena have not been observed to date. We derive the laws of refraction for
baseband, X-wave, and sideband ST wave packets that reveal fascinating
refractive phenomena, especially for the former class of wave packets. We
predict theoretically, and confirm experimentally in the accompanying papers,
refractive phenomena such as group-velocity invariance (ST wave packets whose
group velocity does not change across the interface), anomalous refraction
(group-velocity increase in higher-index media), group-velocity inversion
(change in the sign of the group velocity upon refraction but not its
magnitude), and the dependence of the group velocity of the refracted ST wave
packet on the angle of incidence.
