Each half wavelength has been numbered, so we can keep track of it. In butterflies, the thin-film optics are visible when the wing itself is not covered by pigmented wing scales, which is the case in the blue wing spots of the Aglais io butterfly. Light that is transmitted reaches the bottom surface and may once again be transmitted or reflected. The name is a bit misleading, because the structure in the pattern observed is dominated by interference effects. i [5], The first production of thin-film coatings occurred quite by accident. 0 He noticed that pressing the barbule would shift the color toward the blue, while swelling it with a chemical would shift it toward the red. Figure A shows a wave incident on a thin film. The oil may have an index of refraction near 1.5 and the water has an index of 1.33. l 71 0 obj
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( This phase shift occurs if the refractive index of the medium the light is travelling through is less than the refractive index of the material it is striking. Because a half wavelength fits in the film, the peaks of one reflected wave line up precisely with the troughs of the other (and vice versa), so the waves cancel. g . Part of the wave is reflected off the top surface of the film; note that this reflected wave is flipped by 180°, so peaks are now troughs and troughs are now peaks. θ m . The thickness of the film is very small at the point of contact and gradually increases from the centre outwards. Thus, the film shown in these figures reflects more strongly at the wavelength of the light beam in the first figure, and less strongly at that of the beam in the second figure. A wave is reflected in phase when it tries to enter a medium with a faster speed of light (when passing from a medium with a higher index of refraction to a medium with a lower index of refraction). %%EOF
��Q���{��i�]7t�mG��8ɳ7\��97�����U��j��(;pS��x�����ZlHU��î�v� ��Ôv�-�K_�v�H����' ( is the film thickness, Thin Film Interference When light hits a surface, it can be absorbed, transmitted or reflected. !���*HN=o�ܣv��!=�l��c���xW�}��~S�,���v�W��б�֢��6�Jq[�Um���M%a�0��U9�K���U��ܕ��CRo�����FPM��=�JV;�� ȏ�$lX��}b�?����*eʿ��ᮍ�j� �ըJ#�|\�iA'Km�Ϳ�O�/4�j� > Young's contribution went largely unnoticed until the work of Augustin Fresnel, who helped to establish the wave theory of light in 1816. = n w
If, on the other hand, the reflected beams have opposite phase, the resulting beam is attenuated (as in the second figure). For waves incident normal to the film, the path length difference is twice the film thickness (, For waves incident at an angle to the normal, the path length difference is longer by a factor of, A path length difference equal to a whole number of wavelengths (, A path length difference equal to an odd number of half wavelengths (. i If the distance traveled by beam A is an odd integer multiple of the half wavelength of light in the film, the beams destructively interfere (as in the second figure). Additionally, a phase shift of 180° or Various possible film configurations and the related equations are explained in more detail in the examples below. Interference of transmitted light is completely constructive for these films. t This means none of the wave energy is reflected back into the first medium. {\displaystyle n_{\rm {air}}
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