Pellin-Broca prism
A Pellin-Broca prism is a special dispersion prism ( prism that separates different wavelengths). It is used to select a line from laser light sources that emit a line spectrum with several lines. The light beam is deflected by 90 ° by a Pellin-Broca prism.
Structure and functionality
A Pellin-Broca prism consists of a glass body with a special geometry. The glass body can be thought of as a combination of three individual prisms: two 30 ° -60 ° -90 ° prisms (the angle specifications correspond to the inner angles of the prism) and a 45 ° -45 ° -90 ° prism.
A collimated light beam takes the following path in the prism. As shown in the adjacent figure, light falls on the AB side of the prism. With glass as the prism material, according to the Fresnel equations, only a small proportion (approx. 4%) is reflected at the interface and the rest (approx. 96%) is refracted with minimal deflection according to the law of refraction . Inside the prism, the light beam initially runs parallel to the prism base AC and hits the side BC at an angle of incidence of 45 °. The light is totally reflected due to the selected prism material and rotated by 90 °. The beam now runs further in the direction of the DC side, where it exits the prism again taking into account the minimal deflection.
The selection of the wavelength, as indicated via the "minimum deviation" ( English minimum of deviation ). This is understood to mean the case in which a light beam passes through a prism in such a way that the same angles occur on the two refracting surfaces. In the case of a normal (isosceles) prism, the incident beam and the beam emerging from the prism are rotated by the angle of intersection of the two refracting surfaces; this angle of intersection with the Pellin-Broca prism is 90 °. The effect of the minimum deflection, however, depends on the angle of incidence and on the refractive index of the prism. Because of the dispersion (change in the refractive index of a material as a function of the wavelength), only a special wavelength is deflected minimally, i.e. exactly by 90 °. With the help of a slit diaphragm you can filter out the beam deflected by 90 °. If the prism is now rotated perpendicular to the image plane, the minimum deflection results for a different wavelength due to the changed angle of incidence and the dispersion of the prism material. The desired wavelength can be selected by simply rotating the prism without further changes to the optics.
Another special feature is that a beam of light at the Brewster angle falls on the prism, again leaving this at the Brewster angle. When passing through the Pellin-Broca prism, the light is therefore polarized parallel to the plane of incidence , with almost no reflection losses (based on the parallel polarized portion). In this case the Pellin-Broca prism is also a Brewster prism .
Individual evidence
- ↑ Eugene Hecht: Optics . Oldenbourg Wissenschaftsverlag, 2005, ISBN 978-3-486-27359-5 , p. 310 .