- Along x i.e. parallel to the interface
y component of
and z component of 
are in phase (both time and space) and propagate with phase velocity
.............................(6.78)
The wave propagating along the x direction has its amplitude varying with z and hence constitutes a non uniform plane wave. Further, only electric field 
is perpendicular to the direction of propagation (i.e. x), the magnetic field has component along the direction of propagation. Such waves are called transverse electric or TE waves.