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:
- I 1 – point of incidence on the 1st face
- I 2 – point of incidence on the 2nd face
- n 1 – refractive index of the medium where the blade is immersed
- n 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:
