3:38 AM Tue May 30 I I I 55' L 100% I&) +5. ., ,, WW , 0/13,,9/ a Physrcs 7' Lab (Total mtemal reection and Plund s measurement of the refractive index of glass) Light will generally refract when it passes across an interface. There are. however, some special circumstances in which this will not occur. Il'the light begins in a medium with a higher refractive index than the medium it aims to pass into. then there will be some critical angle of incidence beyond which no refracted ray will appear. This phenomenon is called total internal reection. In order to calculate the critical angle for total internal reection simply set 9m\": in the law ofrefraction to its maximum possible value. Then solve for t9JIICr 9~ 1. What is the critical angle for total internal reection for an air-water interface? Test this. 2. Pfund's method for measuring refractive index: a narrow beam of light passes through the top face of a glass plate. then undergoes a diffuse reection from the rear surface of the plate which has been painted white. The difise light travels back toward the face. Part of this light passes out through the face, and part undergoes total internal reection and reects back to illuminate the rear surface again. When viewed from above, the result is a very bright spot within a dark circle surrounded by a bright halo. a) Measure the glass thickness with a caliper and the dark circle diameter with the ruler. t= ic m D = 3c ,1 b) Use the diagram to relate the critical angle to t and D. From this calculate nglm. Assume nm = 1. - .1 .7 r- _ ~ F __ - 6 Jan (.072). 34 u ? (/9) - Q75 / l / I] 51/! 3 6 *' /_ 00 5 11- f O "{EEJJAryAa