Refractive index

Refractive index

1. REFRACTIVE INDEX, OPTICAL ROTATION & DISSOCIATION CONSTANT By SURYAKANT VERMA Assistant Professor, Department of Pharmaceutics, Dr. K. N. Modi Institute of Pharmaceutical Education & Research, Modinagar , U.P., India.

2. Refractive Index Light passes more slowly through a substance than through a vacuum. As light enters a denser substance, the advancing waves at the interface are modified by being closer together owing to their slower speed and shorter wave-length. If the light enters a denser substance at an angle, one part of the wave slows down more quickly as it passes the interface, and this produces bending of the wave toward the interface. This phenomenon is called refraction. The relative value of refraction or bending of light between two substances is given by the refractive index, n:

3. sin r R efractive index n = sin i Refractive index n = velocity of light in first substance velocity of light in second substance In which sin i is the sine of the angle of the incident ray of light and sin r is the sine of the angle of the refracted ray. Normally the numerator is taken as the velocity of light in air, and the denominator is the material being investigated.

4. R e fr ac t i v e i nde x ?? ? • The refractive index or index of refraction of a substance is a measure of the speed of light in that substance. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium. A simplified, mathematical description of refractive index is : n = velocity of light in a vacuum / velocity of light in medium Hence, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in a vacuum than it does in water.

5. Why Refractive index is Important ? The refractive index of a material medium is an important optical parameter since it exhibits the optical properties of the material . Its values are often required to interpret various types of spectroscopic data . Such as DOAS, ES, RPFTS, IRRS,PWMS . It is used to calculate the focusing power of lenses , and the dispersive power of prisms. The refractive index coefficients are important parameters in the design of a solid state laser . The adulteration problem is increasing day by day and hence simple, automatic and accurate measurement of the refractive index of materials is of great importance these days.

6. Refractive Index (RI) Detector is designed for high-performance liquid chromatography (HPLC) applications. It provides sensitivity, stability, and reproducibility for the analysis of components with limited or no UV absorption Various methods and techniques for the measurement of refractive indices of liquids, solids and gases. Sensitive determination of the refractive indices of certain materials is very important in many fields of research such as material analysis and environmental pollution monitoring. Refractometric measurements are used for qualitative analysis for different type of packaging material like glass, plastics, rubber, silk etc.

7. air g l a ss nor m a l in cide nt ray angle of i n c i de n ce re frac ted ray Refraction is the bending of light when the light passes from one medium to another Useful words to describe refraction of light. i r angle of r e f r ac t i o n

8. i . e . sin i sin r = constant When the ratio of the sin i to sin r is constant . Laws of refraction – This is called Snell’s law . Snell's law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of velocities in the two media, or equivalent to the opposite ratio of the indices of refraction.

9. the law is used in ray tracing to compute the angles of incidence or refraction, and in experimental optics and gemology to find the refractive index of a material v = velocity, SI units are m/s n = refractive index, which is unitless

10. Refractive index and speed of light dist ance tha t lig ht t rave l in 1 s vacuum (or air) 3  10 8 m water ( n = 1.33) 2.25  10 8 m glass ( n = 1.5) 2  10 8 m diamond ( n = 2.42) 1.25  10 8 m

11. Optical Rotation

12. Optical rotation  or  optical activity  (sometimes referred to as  rotary polarization ) is the rotation of the plane of  polarization  of  linearly polarized  light as it travels through certain materials . Optical activity occurs only in  chiral  materials, those lacking microscopic mirror symmetry. Unlike other sources of  birefringence  which alter a beam's state of polarization, optical activity can be observed in  fluids . This can include gases or solutions of  chiral molecules  such as sugars, molecules with helical  secondary structure  such as some proteins, and also  chiral liquid crystals . It can also be observed in chiral solids such as certain crystals with a rotation between adjacent  crystal  planes (such as  quartz ) or  metamaterials . Rotation of light's  plane of polarization  may also occur through the  Faraday effect  which involves a static magnetic field, however this is a distinct phenomenon that is not usually classified under "optical activity."

13. • Polarimeters are optical instruments for measuring the rotation or ‘twisting’ of light. Industrial and academic laboratories use polarimeters for a variety of purposes ranging from simple quality control to fundamental research into complex chemical structures.

14. • A sample tube containing the test liquid (solution) is placed between two polarising elements (polaroid strip or a calcite crystal). The first element, the polarise r , polarises the light before it passes into the sample. The second element, the analyser , can be rotated to counteract any rotation by the sample and hence locates the resultant angular position of the light plane and hence the amount of rotation caused by the sample.

15. Optical Activity When certain organic liquids, Solutions (like suger) or quartz crystal are placed in the path of plane polarized light, the plane of polarization is rotated. The property by vertue of which the plane of polarization of light is rotated called optical activity and the substance possessing this property are said to be optical active . The substance which are rotate the plane of polarized light towards the right (Clockwise) are called DEXTRO-ROTATARY (+) while which rotate towards left (anticlockwise) called LAEVE-ROTATARY (-). A mixture of these two varieties in equal proportion will optically inactive called racemic mixture

16. The magnitude of rotation depends upon the following factor Nature of substance . Length of liquid column (l) through which light passes. Concentration of solution Nature of the solvent Temp of the solution Wavelength of light is used.

17. • • The rotatory power of given solution is generally expressed as specific rotation. It is the number of degree of rotation of the plane polarized light produced by one decimeter in length filled with a solution having one gram of substance per ml. the measurement is carried out at a temperature t using sodium light. The specific rotation can be calculated by following relation. α= observation angle of rotation L = Length in decimeter C = Grams of substance in 100 ml of solution

18. Application of optical Activity and optical rotation A convenient method for study of optical isomerism, configuration and conformation of compound . If the specific rotation of the sample is known, the concentration of optically active substance in the solution can be calculated. Alternatively . If the concentration of the material in the sample is known, the specific rotation can be calculated and used for identification purpose . Quantitative methods of analysis by polarimetry are widespread particularly for plant control in the pharmaceutical industry. The most extensive application in the sugar industry . In other applications, for example R&D, the user may be required to be fully conversant with the principles and to be in a position to investigate optical properties of test materials, possibly for the first time .

19. a quantity expressing the extent to which a particular substance in solution is dissociated into ions, equal to the product of the concentrations of the respective ions divided by the concentration of the undissociated molecule . In  chemistry ,  biochemistry , and  pharmacology , a  dissociation constant   K{d} specific type of  equilibrium constant  that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a  complex falls apart into its component  molecules , or when a  salt  splits up into its component  ions . The dissociation constant is the  inverse  of the  association constant . In the special case of salts, the dissociation constant can also be called an  ionization constant . DISSOCIATION CONSTANT

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