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 § 2 of the Fresnel zone. The straightness of light Evaluation of the integral formula (2) is generally difficult. Fresnel solved the problem of finding the amplitude in point P, replacing integration by summation, ie switched from continuous sums () to digital (Σ). A point source S generates a spherical wave. Required to determine the amplitude of vibration waves in the in point P. Wave surface at a point O on the sphere will be. Fresnel proposed to split it into annular zones (sectors) so that the distance from the edges of each zone to point P is different on λ/2. Thus constructed sector areas called zones Fresnel. Waves coming in point P from similar points of two neighboring zones have opposite phases, as path difference between the waves is λ / 2. Is not too large m (m - number of zone), the area of the Fresnel zones are approximately equal S1= S2 =Sm. With the increasing number of the zone m distance increases bm, from zone to point of P and the angle φ between the normal. to the elements of the zone and the direction to point of P. Then by (1) the amplitude of fluctuations Am excited m-th-zone in point P, decreases monotonically А1 >A2 >A3 >Am >…>A∞. Because waves from two adjacent areas come in point P in opposite phase, they cancel each other, and then the resulting amplitude in point P is   Арез = А1 -A2 +A3 –A4+… Because Am decreases monotonically, we can assume And Аres can be written as If the wave front is completely open, the number of zones m → ∞ and Amplitude, created at some point P across the surface of a spherical wave is half the amplitude of the first zone. Consequently, the distribution of light from S to P is as if the light output is distributed in a very narrow channel along the SP. that is straightforward. Zone plates are used to enhance the intensity of light in the so-P by the overlap of even (or odd) Fresnel zones - amplitude zone plates, or changes on phase of the wave on π, passing through the thicker-even (odd) parts of the plate - phase zone plates.                 If the paths of light waves to put a screen with an opening into which is placed an even number of Fresnel zones, in point P is a minimum - light loss: If the hole is laid odd Fresnel zones, in point P is a maximum - light amplification   3 the Fresnel diffraction Fresnel diffraction or diffraction of spherical waves is, if the diffraction pattern is observed at a finite distance from the obstacle. 1. Diffraction by a circular aperture                                  r0 –the radius of the hole. at r0 <