A theory is developed to explain the origin of physical phenomena in quasiwaveguide, based on the forward scattering and multiple‐wave interference effects. The properties of quasiwaveguide modes, such as positions, linewidths, the reflectivity of the reflected light spot, and the reflectivity distribution of scattered light are described. The shape of them‐lines is derived theoretically. Applications of the theory are also discussed. ©1996 American Institute of Physics.