Dispersive prism

Dispersion prisms are a group of optical prisms, whose function utilizes the wavelength dependent refraction (see dispersion) of light at the entrance and exit faces of the prism. They are used inter alia for the production of light spectra, for example, a prism spectrometer.

Functioning and types

The easiest and most frequently used form of a dispersion prism, an optical prism having a triangular cross-sectional area. If a beam of light on the interface of air and the prism, the light beam is due to the different propagation velocity of the light refracted in the media, that is, not at normal incidence on the interface of the light beam undergoes a certain deflection. Due to the wavelength-dependent index of refraction (similar to the velocity of propagation ) of materials undergoes in a collimated beam of light at each wavelength multicolored another deflection and the refracted light propagates as a divergent beam from the light.

This effect is analogous to and at the exit surface of the light rather than the steel, which leads to the fact that in parallel to the inlet and outlet surfaces, and the beam divergence splitting cancels. In a dispersion prism entrance and exit faces are inclined to each other in shape so that it comes after the passage of a divergent bundle of rays and spectral splitting.

In addition, there are a variety of other geometries can be used as a dispersion prism. Among them are prisms in which the light beam is reflected at one or more surfaces ( metallic reflection and total reflection ), for example when Littrow prism or the Pellin - Broca prism.

Dispersion prisms special design are for example:

• The vision prism ( which is often referred to himself only as a dispersing prism): sequencing of simple triangular prisms with different material properties,
• Pellin Broca prism - the: square prism with an additional total internal reflection for 90 ° deflection of the diverging light beam and suitable as a Brewster prism for lossless beam deflection linearly polarized light.

Applications

Monochromator

In spectrometers dispersion prisms are used to produce a constant, minimal deflection for a particular wavelength. By the spectral splitting of the light in a specific wavelength can be chosen according to the passage through the prism, for example, a slit. By rotation of the prism in the cross- section plane, also allows the wavelength of light with minimal deflection change, and therefore, as a monochromator ( prism monochromator ) can be used. Examples are the Littrow prism and the Pellin - Broca prism.

Similarly function dispersion prisms modern spectral ellipsometers are used to achieve shorter response times for a spectrum is not generated first monochromatic light with them, that is subsequently irradiated on the sample but more colored light after being reflected by the sample in a prism spectrally split and the individual colors on a CCD line measured at the same time.

Deflection monochromatic light

The dispersion of the light, that is, its refractive index varying with the wavelength to determine the passing of a material that distractions different monochromatic light is measured. A beam of light enters such a definition, not dissected from the prism and unaufgeweitet out. His exit angle and thus its deflection are clear and easy to identify.

With corresponding measurements of the symmetrical light transmission is applied, wherein the deflection is a minimum and writable by the following simple equation:

Where: n = refractive index of the material for the predetermined wavelength of light

The minimum deflection is easily recognizable and relatively safe. An appropriate measuring device is a goniometer spectrometer.

Combination of individual dispersion spectra

By combining two or more dispersion prisms can be in addition to the spectral splitting of the light realize more functions, such as a total achromatic or a non- distracting for a certain wavelength behavior.

An achromatic behavior on a prism means that there is after the passage of the assembly no angular dispersion for different wavelengths, that is, the different color light beams do not diverge further, but parallel to each other. This can for example be realized by the combination of a 60 ° prism of crown glass and one half 60 ° prism of flint glass. A dispersion prism having this effect is referred to as achromatic prism.

Other combination, however, can be used as a direct-vision prism. In this type of dispersion prisms a tiny distraction for a certain wavelength is achieved. However, the angular dispersion is maintained. A typical arrangement is the sequencing of simple triangular prisms with different materials, such as crown and flint glass.