In optics, an Abbe prism, named for its inventor, the German physicist Ernst Abbe, is a type of constant deviation dispersiveprism similar to a Pellin_Broca prism.
The prism consist of a block of glass forming a right prism with 30°-60°-90° triangular faces. When in use, a beam of light enters face AB at point O, is refracted and undergoes total internal reflection from face BC, and is refracted again on exiting face AC. The prism is designed such that one particular wavelength of the light exits the prism at a deviation angle (relative to the light's original path) of exactly 60°. This is the minimum possible deviation of the prism, all other wavelengths being deviated by greater angles. By rotating the prism (in the plane of the diagram) around point O, the wavelength which is deviated by 60° can be selected.
The dispersive Abbe prism should not be confused with the non-dispersive Porro-Abbe prism.
The Abbe 5 is an affordable refractometer ideally suited for use where a wide refractive index measurement range is required such as in small contract laboratories or applications where sample throughput is relatively low.
The sample is placed on the prism and is illuminated by aligning the chrome-plated reflector with a suitable light source (day light or desk lamp – not provided).
The Abbe 5 refractometer is supplied with a glass calibration plate, contact liquid, and instruction manual and is packed in a hard flight case for easy transportation and storage.
The simplest are really spectrometers, consisting of a glass prism, usually hollow and fitted with accurately parallel glass sides, mounted on a table which carries a fixed collimation tube and a movable observing tube, the motion of the latter being recorded on a graduated circle.
The refracting angle of the prism, i in our previous notation, is determined by placing the prism with its refracting edge towards the collimator, and observing when the reflections of the slit in the two prism faces coincide with the cross-wires in the observing telescope; half the angle between these two positions gives i.
The prisms, which are right-angled and made of the same flint glass, are mounted in a hinged frame such that the lower prism, which is used for purposes of illumination, can be locked so that the hypothenuse faces are distant by about 0.15 mm., or rotated away from the upper prism.
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