The physical path a signal or signal component follows as it propagates through a waveguide. Some signal components travel directly through the center of the waveguide, at least theoretically, and, therefore, travel the shortest possible distance between the point at which they enter the waveguide and the point at which they exit the waveguide. Other modes take more transverse paths, striking and reflecting back and forth off of the interface between the core and cladding as they propagate through an optical fiber, for example. Low-order modes take modestly transverse paths, while high-order modes take considerably more transverse paths. Some modes at the transmitter can be so transverse as to strike the corecladding interface at less than the critical angle and, therefore, penetrate the interface and be permanently lost in the cladding. Figure M-4 illustrates the differences between these paths. See also cladding, core, critical angle, and waveguide.