. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. Fig. 1. DIAGRAM TO ILLUSTRATE REFRACTION. ever great or sniall the !ino-l(> of incidence may ])e, there a constant relation between it and the angle of refraction for two given substances. This constant relation is the ratio between the sines of the incident and refracted angle, and is called the index of refractiooi. When a ray of light passes from one medium into another which is less refracting, as from water into air, the
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. Fig. 1. DIAGRAM TO ILLUSTRATE REFRACTION. ever great or sniall the !ino-l(> of incidence may ])e, there a constant relation between it and the angle of refraction for two given substances. This constant relation is the ratio between the sines of the incident and refracted angle, and is called the index of refractiooi. When a ray of light passes from one medium into another which is less refracting, as from water into air, the angle of incidence is less than the angle of refraction. Hence when light is propagated in a mass of water there is always a value of the angle of incidence such that the angle of refrac- tion is a right angle, in which case the refracted vax emerges parallel to the sur- face of the water. This angle is called the critical angle, since for any greater angle the incident ray can not emerge, but undergoes an internal reflection, which is called total reflection because the incident light is entirely reflected. From water to air the critical angle is 48° '. In th(^ example given, air and water, r = 48° 35'. Now, sup- posing the light to go from 1> to r>, the line oc will coincide with the line of (the critical angle). If the value of /• is increased, the ra}^ will no longer pass from water into air, 1)ut undergoes total reflection at the surface o. In total reflection there is no loss of light from absorption or trans- mission, and accordingly it produces the greatest brilliancy. The luster of trans- parent bodies bounded b}^ plane surfaces, 'i such as the luster of gems, arises mainly from total reflection. This luster is the more frequent and the more brilliant the smaller the limiting angle. The diamond, having the smallest value for its limiting angle, is the most brilliant of all gems. There are certain transparent substances which possess the power of splitting
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