Parallel faced blade

The blade is the association of two parallel plane diopters: air/glass and glass/air

The Parallel Faced Blade is a system of three homogeneous and transparent media separated two by two across flat, parallel surfaces. Of the three media, typically the second media is the parallel-faced blade. As an example, one can cite a pane of glass in a window.

A monochromatic ray of light, when falling obliquely on one of the faces of the blade, crosses it, emerges from the other and undergoes a lateral deviation d . The second half being the blade, if the first and third half are equal, the incident ray will be parallel to the emerging one; if the first medium is different from the third, the incident ray will not be parallel to the emerging one.

The lateral deviation d is obtained geometrically through the figure below:

  • 1 – point of incidence on the 1st face
  • 2 – point of incidence on the 2nd face
  • 1 – refractive index of the medium where the blade is immersed
  • 2 – refractive index of the material that makes up the sheet
  • d – lateral deviation suffered by the radius
  • e – blade thickness
  • θ = i – r

By the Snell-Descartes Law, we have:

Dividing member by member, (I) and (II):

As we have that θ = i – r, then:

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