Prism

Under a prism in the optical system, a component in the form of the geometric body understood prism that is used for different optical effects, for example as a dispersive ( light according to wavelengths auffächerndes ) element or to deflect a light beam. In the mold, it frequently is a right prism with a triangle as a base surface. The optical properties of the prism depends substantially on the another triangle angles or the position of the optically active prism surfaces and the refractive index of the material ( for example, glass or transparent plastic).

Applications

The main applications of the prism are typically based either on his capacity to refract light wavelength dependent - that is then " dispersion" - or on redirect light rays by reflection on one or more surfaces, such as in the viewfinder of an SLR. Both total reflection and specular reflection arrive with metal -coated prism surfaces are used.

Dispersion prism

Prisms in which the light- wavelength-dependent (" dispersive " ) behavior of the prism material is used are referred to as dispersion prisms. They are used for example in a prism spectrometer. Here, the light is incident obliquely on a side surface of the prism, in the simplest case a prism in the form of an isosceles triangle. Due to the oblique (at least not perpendicular ) incidence and the wavelength-dependent refractive index of the incident light beam is broken, depending on the wavelength at a different angle in the prism. This effect increases again at the exit face of the prism, where the split light beam is incident at different angles of incidence. The light then falls onto a screen, the spectrum of the light source is visible. The common edge of the two surfaces concerned ( in the chart above), the deflecting means or refracting edge, relative to which the light is deflected in the opposite direction.

Conversely, this effect with the use of monochromatic light of known wavelength to be used for determining the refractive index ( of the dispersion). This exploits that the deflection of the diffracted light is minimal for a symmetric crossing. The refractive index can then be determined as follows:

With:

Reflection prism

Prisms, whose application is based on the effect of total reflection are used for low-loss deflection of light. They are called deflecting prism and reflecting prism. The simplest example of a deflecting prism is a prism with the base of an isosceles right triangle. In this case, the two mutually perpendicular surfaces form the inlet and outlet surfaces. The third surface serves as a reflection or deflection, at which incident light at an angle greater than the critical angle of total reflection is totally reflected, i.e. without reflection losses. To achieve the highest possible transmittance of the entire optical path, the light is guided at right angles to the entrance face. In this case, the transmittance for light unpolarisertes maximum and to avoid a splitting of the light depending on the wavelength or the polarization, perpendicular to an interface for incident light is not refracted. Simple deviating prisms are used in optical instruments specifically for guiding light, for example, for low-loss deflection of a laser beam in FTIR spectrometers.

Combining two reflecting prisms, the mirror image effect of a single reflection can be reversed and an image is raised in this way. Such, also referred to as a reverse prism systems are used for example in prism binoculars. Also the Penta prism with a pentagonal base as, in principle, such a combination of two triangular reflecting prisms. It is used for 90 ° deflection, the image remains the right way. At Penta Roof Prism one of the four reflective surfaces is replaced by two mutually perpendicular surfaces, whereby the image is mirrored. It is used in SLR cameras, to correct the wrong by their mirror image.

The advantages of a totalreflekierenden deflecting prism against a mirror arrangement include the Lageunveränderlichkeit of the reflecting surfaces, the compact construction and the higher transmittance at several reflections. So the Transmissonsgrad a penta prism is significantly higher than that of a penta mirror, because the prism occur only reflection losses at the entrance and exit to ( in glass, is 4 %), which can be reduced with appropriate anti- reflection layers again. The reflection losses at a metallic surface, however, be about 6 to 10%, resulting already in two reflections at significantly lower transmittances of the arrangement.

Polarizing prism

Polarizing prisms lend themselves to separate light into components of different polarization. Their operation is based on the birefringence property of the prism material used. Unlike dispersion prisms not dependence of the refractive index on the wavelength, but its dependence is exploited by the polarization of the light. That is, incident light is refracted to different degrees depending on the polarization, and the beam portions diverge in the prism. Not primarily determines the position of the boundary surfaces to one another, but the position of the boundary surfaces of the optical axis of the birefringent material used the properties of the polarization prism. To reinforce this splitting or selection of the polarization is a combination of two differently oriented prisms part is generally used, which are often stuck together, the adhesive (such as Canada balsam ) further having an optical purpose. Often they are joined at an angle to the original beam direction of a running surface. Due to the oblique incidence and the different refractive index can thus be realized for example by total reflection a selection.

One of the most polarizing prisms is the Nicol prism. Variants are the Glan-Thompson prism and Glan -Taylor prism.

Prism diopter

As a measure of the strength of a prism, the unit was previously used prism diopter ( unit = pdptr ) is used. Today, they still appeared occasionally in ophthalmology. It is defined by the degree of deflection of a light beam measured in centimeters in one meter distance (cm / m). Accordingly, one pdptr the deflection of a light beam to 1 cm in 1 m distance. For a description of prismatic effects only the units of centimeters per meter should be used (cm / m ) (1 pdptr = 1 cm / m).