Medical brain notes 14

  INTRODUCTION Light passes more rapidly through a vacuum than through a substance (medium). It has been observed that when a ray of light happens to pass from one medium ( a ) into another medium ( b ) it is subjected to refraction (Figure 18.1). In other words, the ray travels at a lower velocity in the relatively more optically dense medium ( b ) than in medium ( a ) which is less optically dense. It is a common practice to compare the refractive indices of liquids to that of air. According to Snell’s Law we have :         .....................(1) where,  i  = Angle of incidence,   r  = Angle of refraction, and   n  = Refractive index of medium ( b ) relative to medium ( a )   Critical Angle  vis-a-vis  Refractive Index   Figure 18.2, represents the critical angle which is used invariably in refractometry. Considering a narrow band of rays,  x - y , held near to the boundary between the two media ‘ a ’ an...

  THEORY Lorentz and Lorentz in 1880, introduced the terminology  specific refraction  or  refractivity  which may be expressed as :                 .............................(3) where,  n  = Refractive index,   p  = Density of the substance*   Hence, the specific refraction (Eq. 3) is considered to be a more useful property and is characteristic of the substance, being absolutely independent of temperature.   Molar Refractivity :  Later on, a still more useful property termed as the molar refraction (or refrac-tivity) was introduced which could be expressed as follows :              .............................................(4) where, R = Molar refraction,   P = Density of the substance, and   M = Molecular weight.   Interestingly, both specific refraction [ n ] and molar refraction (R), being temperature independent, should have th...

  INSTRUMENTATION In Figure 18.3, the optical system of  Abbe’ Refractometer  has been shown based on the critical angle principle. The various parts in Figure 18.3, are stated as below :   A = Mirror ;                                                  F = Telescope ; B = Liquid (Sample) ;                                   G = Scale ; C = Prism box ;                                   ...

  DETERMINATION OF REFRACTIVE INDEX OF PHARMACEUTICAL SUBSTANCES A large number of pharmaceutical substances such as volatile oils, namely : peppermint oil, lemon oil, aniseed oil have a definite range of refractive index. Based on this physical characteristic it is possible to ascertain the purity of this volatile oil precisely and accurately.   Materials Required :  Abbe’ refractometer, volatile oil, xylene, capillary tubes ;   Procedure :  In order to obtain precise and accurate measurements the prism case of Abbe’ refractometer   is attached to a thermostat bath whose temperature is previously maintained at 25°C. Open the prism box gently and place a few drops of pure volatile oil on the lower prism with the help of a capillary tube and finally close the box. The mirrors are duly adjusted so as to obtain a bright illumination of the field of view. The knurled knob is turned gradually until the field of view displays a dark and light zone. In case, a col...