We report a phenomenon of spectral anomalies in the interference field of Young's double-slit interference experiment. The potential applications of the spectral anomalies in the information encoding and information transmission in free space are also considered.

A simple method for measuring the refractive index of liquid is presented. When a laser beam impinges obliquely on a rectangular cell filled with liquid and passes through the cell, the propagation axis of the transmitted beam is displaced from that of the incident beam. By measuring the displacement, we can determine the refractive index of the liquid. Beams of a He-Ne laser and a laser diode were used for measuring the refractive indices of pure water and some organic liquids.

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When the waist size of a Gaussian beam becomes of the order of the wavelength of light, the beam does not satisfy the paraxial condition in which it is derived. In this paper, we define the lower bound to the waist size by showing that a Gaussian beam whose waist size is larger than this bound safely satisfies the paraxial condition. A beam which is Gaussian in form but violates the paraxial condition is called a nonparaxial Gaussian beam. We clarify the range of the waist size for which the first-order correction to this beam is effective. It is shown that a distinct value of the waist size exists for which the paraxial approximation completely fails and the first-order correction never works.

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We present the detailed behavior of the axial force acting on a dielectric sphere exerted by the optical pressure of a focused Gaussian laser beam. Comparison is made between the numerical results and those calculated from the radiation pressure cross section. There is also a discussion as to whether the expressions for the axial force given in this paper are consistent with the previously reported experimental results. Moreover, a simple experimental method to measure the axial force on a polystyrene sphere suspended in water is demonstrated, and fairly good agreement between theoretical and experimental results was obtained.

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A gradient-index rod whose length is a quarter of the periodic length works as a retroreflective element when its endface is coated with metal. This paper describes the experiment and theory on the reflectivity of such a rod and that of an array comprising seven or nineteen rods when they are illuminated by a Gaussian laser beam. We clarify how the reflectivity depends on the offset of the beam axis and the inclination angle of the axis. The far field pattern of the reflected wave is also investigated.