Glan-Thompson prism

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The Glan-Thompson prism (after Paul Glan and Silvanus Phillips Thompson ) is a polarizer based on birefringence and total reflection that linearly polarizes unpolarized light (s-polarization, i.e. the plane of polarization is perpendicular to the plane of incidence). The principle was introduced by Paul Glan in 1880 and improved by Silvanus Philipps Thompson in 1881.

construction

Schematic representation of the beam paths in a Glan-Thompson prism

The Glan-Thompson prism, like the Nicol prism developed earlier (1828), consists of a birefringent crystal (a typical material is calcite ). The crystal is cut into two right-angled prisms so that its optical axis is parallel to an end face. The two prisms are joined to a cuboid with a transparent adhesive .

The angle of intersection of the two halves of the prism is chosen so that the ordinary beam is totally reflected at the interface while the extraordinary beam is passed through with almost no reflection. For this, the adhesive must have a refractive index that is between that for the ordinary and that for the extraordinary ray of the birefringent material. In the case of calcite at a wavelength of λ = 589 nm between 1.486 (extraordinary ray) and 1.658 (ordinary ray), for example the Canada balsam , which was often used in the past, with n K  = 1.54. Nowadays, materials are used that have better transmission properties in the near infrared, for example crystalline glycose or glycerine .

functionality

When entering a material, the incident light beam is refracted according to Snellius' law of refraction . Due to the anisotropic refractive index in the birefringent material, the incident beam is split into an ordinary (polarized perpendicular to the optical axis of the crystal) and an extraordinary (polarized parallel to the optical axis) beam. Since the outer cut surfaces of the crystal in the Glan-Thompson prism are parallel to the optical axis, both rays are refracted equally at normal incidence - due to the normal incidence, the angle of incidence is equal to the angle of refraction, i.e. 0 °. The two parts of the beam move on a common path in the crystal, albeit at different speeds because of the different refractive index.

If both rays hit the inner cut surface, they are reflected differently due to the appropriately chosen materials. Compared to calcite, the adhesive is an optically thinner medium for the normal beam. It is therefore totally reflected at the cut surface, so that it hits an outer surface of the Glan-Thompson prism, is broken and exits or is absorbed by an absorber that may be placed becomes. Calcite is the optically thinner medium for the extraordinary jet. It therefore penetrates the cut surface almost undisturbed according to the Fresnel equations (T ≈ 99.9%). This is repeated when crossing into the second half of the prism. Care must be taken that the angle of intersection (and thus the angle of incidence) is not too large, as otherwise the beam will be totally reflected at this interface . When crossing the adhesive layer, the beam experiences a beam offset, but this is only minimal due to the small thickness of the adhesive. When exiting the crystal there is therefore only the linearly polarized, extraordinary beam, the polarization of which corresponds to the alignment of the optical axis.

The emerging beam is largely but not completely polarized. The degree of polarization depends not only on the manufacturing tolerances, but also on the materials used, which are generally not ideal dielectrics, and also on the thickness of the adhesive layer. Since a light beam penetrates somewhat into the subsequent optically thinner material (in the form of an evanescent wave) due to the continuity conditions of the Maxwell equations during total reflection, the so-called optical tunnel effect ( prevented total reflection ) can occur with very thin layers of the adhesive . Depending on the layer thickness, part of the actually totally reflected radiation gets into the second half of the prism. The emerging beam is therefore never one hundred percent linearly polarized. However, since the influence of this effect decreases exponentially with the layer thickness, it can be almost completely reduced by a sufficiently thick layer (> 10 µm).

Differentiation from other types of prisms

This principle is essentially the same as that of the Nicol prism, one advantage is that the aligned cut surfaces of the Glan-Thompson prism mean that there is no offset between the incoming and outgoing beam.

A Glan-Thompson prism that uses an air gap instead of an adhesive is called a Glan-Foucault prism . It is mainly used for high-performance applications for which the Glan-Thompson prism is less suitable, since the glue can heat up and thus be destroyed. Further prisms of the Glan-Thompson type are the Ahrens prism , in which two Glan-Thompson prisms are arranged next to each other, and the Grosse prism , essentially an Ahrens prism with an air gap.

The similarly constructed Lippich or Glan-Taylor prism also allows the extraordinary beam to pass through, but generates a beam that is polarized differently by 90 ° (p-polarized) due to the optical axis rotated by 90 ° in the plane of incidence.

literature

  • Wolfgang Demtröder: Experimental Physics 2: Electricity and Optics . Springer, 2008, ISBN 978-3-540-68210-3 , pp. 255 f .
  • Wilhelm Walcher: Physics internship . 6th edition. 1989, ISBN 3-519-03038-1 .
  • Horst Stöcker: Pocket book of physics: formulas, tables, overviews . 5th edition. German (Harri), 2004, ISBN 3-8171-1720-5 .

Individual evidence

  1. ↑ Paul Glan: About a polarizer . In: Repertory for Experimental Physics, for Physical Technology, Mathematical and Astronomical Instrumentation . No. 16 , 1880, p. 570 .
  2. ↑ Silvanus Philips Thompson: On a new polarizing prism . In: Phil Mag. Band 5 , no. 12 , 1881, p. 349 .
  3. ^ Silvanus Philips Thompson: On the Nicol Prism and its Modern Varieties . In: Proceedings of the Optical Convention . London 1905, p. 216-240 .
  4. ↑ Siegfried Becher: About the astigmatism of Nicols and its elimination in the polarization microscope . In: Annals of Physics . tape 352 , no. 11 , 1915, pp. 285-364 , doi : 10.1002 / andp.19153521102 .
  5. ↑ a b c d Michael Bass (Ed.): Handbook of Optics, Third Edition Volume I: Geometrical and Physical Optics, Polarized Light, Components and Instruments . McGraw-Hill Professional, 2009, ISBN 978-0-07-162925-6 , pp. 13.9-13.12 .