Some pecularities of multiwave light diffraction in optically thin and thick phase unslanted gratings have been considered. An approximate solution of the wave equation is derived by the eiconal method. The solution is valid for gratings which are both optically thin and thick. This is verified by comparison of approximate solutions with numerical calculations. A range of parameters is found where the solution gives a reliable result. It has been found that the phase grating has the property of focusing light in planes with a maximum value of refractive index if the grating thickness is close to the focal distance introduced.
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