Critical Angle

Maximum angle of incidence at which a ray of light will emerge, when meeting the surface of an optically le dense medium. If the second medium be vacuum, the absolute index will be u, the angle of refraction will be 2' and hence u , which is equal to sin r sin i' IMG:content-1031.jpg:[ocr errors] will be sin; the sine of the critical angle is the reciprocal of the refractive index. The critical angle for crown gla is about 41° 48'; that for water about 48° 35'.