  The Dirac cone on a surface of a topological insulator shows linear
dispersion analogous to optics and its velocity depends on materials. We
consider a junction of two topological insulators with different velocities,
and calculate the reflectance and transmittance. We find that they reflect the
backscattering-free nature of the helical surface states. When the two
velocities have opposite signs, both transmission and reflection are prohibited
for normal incidence, when a mirror symmetry normal to the junction is
preserved. In this case we show that there necessarily exist gapless states at
the interface between the two topological insulators. Their existence is
protected by mirror symmetry, and they have characteristic dispersions
depending on the symmetry of the system.
