The e ray gets its name because it does not obey this rule.īirefringence is only a property of solids The ratio of the sine of the angle of incidence to the sine of the angle of ordinary refraction is a constant. ![]() As the name implies, the o ray behaves in an "ordinary" way, following Snell's law without a problem. This phenomenon is known as double refraction or birefringence.Ī ray of light incident on a doubly refractive or birefringent material divides into two rays: an ordinary ray (or o ray or ω ray) and an extraordinary ray (or e ray or ε ray). This can best be seen by laying a large crystal of it on a page of text. It has been known for several centuries that light transmitted through calcite takes two paths. It is of little economic importance by itself (although it is a component of limestone, which is used to make cement), but is of some scientific importance. Nonmetallic minerals that cleave easily were called spar in German and so calcite is sometimes also known as Iceland spar. Pieces of this mineral are easily split (or cleaved as the geologists say) into parallelogram-faced prismatic chunks. It can be found throughout the world, but some of the best samples were originally found in Iceland. Dispersion (the rate of change of index with wavelength) is greater for shorter wavelengths thus, the graph starts out steep and gradually levels off.Ĭalcite is a common, transparent mineral.The index of refraction is larger for shorter wavelengths thus, its slope is always negative.dnįor most transparent materials, a graph of index versus wavelength is curve with a few general characteristics. Dispersion is best quantified as the rate of change of index of refraction with wavelength. Index and wavelength are not linearly related. Use of a single number to quantify dispersion is rather misleading. Where n D is the index of refraction for the yellow D line of sodium at 589.0 nm. Certain sites situated at low elevations on coastlines regularly detect "sea return", a phenomenon similar to ground clutter except that the echoes come from ocean waves."ĭispersion is generally highest in solids and lowest in gases.ĭispersion is often measured in terms of the coefficient of dispersion, which is defined as the difference between the refractive indices for for two prominent lines in the spectrum of hydrogen - the blue F line at 486.1 nm and the red C line at 656.3 nm.Īnother common measure of dispersion is the dispersive power. ![]() This "anomalous propagation" phenomenon (commonly known as AP) is much less common than ground clutter.
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