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Tham khảo tài liệu 'the materials science of thin films 2011 part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 528 Optical Properties of Thin Films nonabsorbing substrate is n2 1.5 the value for plate glass. The oscillatory nature of the reflected light intensity caused by interference effects has a periodicity related to the film thickness and index of refraction. This is the basis for experimentally determining the thickness of transparent films if the index of refraction is known see Chapter 6 . Conversely the optical properties of the film can be determined at a particular wavelength if its thickness is known. Maxima or minima in the reflected intensity occur at specific film thicknesses for given wavelengths depending on whether the refractive index of the film is greater or less than that of the substrate. In the former case the reflectivity is enhanced whereas in the latter case reflectivity is diminished. Optimization of these two effects has led to the development of dielectric mirrors and antireflection coatings respectively. To quantity the issues related to antireflectivity Eq. 11-17 reveals that r vanishes when Tj r2exp ÍÒ 0 or when the denominator goes to infinity. The latter is an impossibility since r and r2 are less than or equal to 1. The remaining condition can be decomposed into two real transcendental equations a r r2cos 0 0 and b r2sin 0 0. Equation b implies that 0 0 ir 2tt 3tt etc. but the simultaneous satisfaction of equation a requires the selection of Ô 7T 3tr 5tt etc. Under these conditions r - r2 0 or n0 - ni - 1 - n2Ì nt n2 - Therefore ni Vn0n2 11-20 Since ổ 4irnỊdỵ X 7T 3iĩ Sir etc X 3Ầ 5X __ n.d etc. 11-21 1 4 4 4 v Equations 11-20 and 11-21 represent the amplitude and phase conditions for zero reflectance respectively. In the design of a one-layer antireflection coating the film index of refraction should be the geometric mean of the refractive indices of adjacent media. This is only strictly true for the wavelength X for which the optical thickness of the film is X 4 3X 4 etc. To coat a glass lens n2 1.52 a film with 7 x 1.52 1.23 is
