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Optical Thin film (Its function and the effect.)
Optical thinfilm is generally configured as a multi layered film. However,to make the explanation simple, single layer anti-reflection coating is taken as an example hereafter.
Light causes reflection when it meets to a boundary of some object,
Concerning a glass substrate which has 1.5 index number, for instance, as illustrated as in the figure below, the light is reflected 4% at the first boundary as incidence side and another 4% is also reflected at the second boundary as emergent side.
Form another point of view, 100% light becomes 92% after passing through (transmits) the glass plate and remaining 8% is reflected as the result. In the dark, by looking outside through an window from blight room, you will see your own. This image is made of the 8% reflection light.
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Such phenomenon is very convenient to look at a beautiful lady stand near by with utilizing such window reflection.
But if such bare glass was applied to a illumination system, the brightness will be lost.
If an optics was the case on the other hand, such reflection will cause ghost or flare.
Also, for light application in optical communication, various problem occurs such as noise generation or shorter propagation distance.
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Here a star, Optical Thin film , comes up. Light behavior is able to control when a transparent film thinner than the wave length of light is put on the glass surface.
For example, when 1.38 index number of Magnecium Floulide film is made on 0.1micron thick glass plate,
its reflection factor is reduced down to 1.41% at each side from 4% of non-coated one.
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Left picture shows a glass plate which left half is anti-reflection coated and the right half is not.
You can see the printed texts of the book better on the coated left half with reduced reflection.
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What makes such effect available is by an interference between the light reflected on the surface of the thinfilm and the light reflected at the boundary of thin film and substrate.
With utilizing this light interference effect by optical thin film, the reflection can be reduced or increased as expected, and becomes available for in use of many purposes.
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