The angle of incidence and angle of reflection of wave fronts vary in size with the frequencies of the input energy.
Arrow shows the direction of propagation.
The cut off frequency in the waveguide is the frequency that causes angles of incidence and reflection to be perpendicular to the wall of guide.
If the frequency is below the cut off frequency, the wave fronts will be reflected back and forth across the waveband and no energy will be conducted down the waveguide.
The velocity of propagation of wave along a waveguide is less than its velocity through free space. This lower velocity is caused by zigzag path taken by wavefront in a waveguide.
The wave propagates down the waveguide in a zigzag manner with the Electric field maximum at the center of the guide and zero at the walls.
Due to this pattern Waves can no longer be TEM because propagation by reflection requires not only a normal component but also a component in the direction of propagation for either the electric or magnetic field, depending on the way in which waves are set up in the waveguide.
The mode with the lowest cutoff frequency is termed the dominant mode of the guide. It is usual to choose the size of the guide such that only this one mode can exist in the frequency band of operation.