Refractive index

Refractive index, Refraction of light, Measurement of Refractive index, Refractometer, Abbe's refractometer

1. Refractive index Mr. V. A. Sansare Asst. Professor of Pharmaceutics. Indira Institute of Pharmacy, Sadavali, Maharashtra.

2. Introduction: Refraction When monochromatic light passes through a less dense medium such as air or vacuum and enters a denser medium, the advancing waves at interface are modified and brought closer together. This leads to decrease in speed and shortening of wavelength .

3. When light passes the denser medium, a part of wave slows down more quickly as it passes through interface and makes it bend towards the interface. This phenomenon is called as refraction . If light passes from denser medium to less denser medium then it is refracted away from the interface.

4. Refractive index The refractive index is a constant for a given pair of materials under specified conditions . It can be defined as ratio of speed of light in material 1 to the speed of light in material 2. This is usually written 1n2 and is the refractive index of material 2 relative to material 1 . The incident light is in material 1 and the refracted light is in material 2 . When the incident light is in a vacuum this value is called the absolute refractive index of material.

5. In other words refractive index of substance is the ratio of velocity of light in vacuum or air to that in the substance . n = Velocity of light in air/vacuum Velocity of light in material

6. Snell’s Law In 1621, a Dutch physicist named Willebrord Snell derived the relationship between the different angles of light as it passes from one transparent medium to another . When light passes from one transparent medium to another, it bends according to Snell’s law which states: Sin i n = Sin r

7. Refractive indices of most substance are more than air because the velocity of light in air is greater than in the substance. Air 1.00029 Crystal 2.00 Water 1.330 Diamond 2.417 Glass, soda-lime 1.510 Ethyl Alcohol 1.36 Vacuum 1.000000 (exactly ) Glass 1.5 Air (STP) 1.00029 Ice 1.309

8. Measurement of Refractive Index: Refractive index is determined by using instrument called refractometer . Abbes refractometer I mmersion refractometer Pulfrich refractometer

9. Abbes refractometer Advantages: Most convenient, reliable and simple instrument. Small sample size requirement. Ordinary light source can be used. Easy maintenance and economy. Easy determinations of RI

10. Components: Light source ( D-line of sodium) Light reflection mirror P rism box (2 prisms) Telescope ( Amici prisms)

11. Principle In the Abbe refractometer the liquid sample is sandwiched into a thin layer between an illuminating prism and a refracting prism. The refracting prism is made of a glass with a high refractive index (e.g. 1.75 ) and the refractometer is designed to be used with samples having a refractive index smaller than that.

12. A light source is projected through the illuminating prism, the bottom surface of which is ground (i.e., roughened like a ground-glass joint), So each point on this surface can be thought of as generating light rays travelling in all directions .

13. Light travelling from point A to point B will have the largest angle of incidence ( θ i ) and hence the largest possible angle of refraction ( θ r) for that sample. All other rays of light entering the refracting prism will have smaller θ r and hence lie to the left of point C . Thus, a detector placed on the back side of the refracting prism would show a light region to the left and a dark region to the right.

14. Samples with different refractive indices will produce different angles of refraction and this will be reflected in a change in the position of the borderline between the light and the dark regions . By appropriately calibrating the scale, the position of the borderline can be used to determine the refractive index of any sample.

15. Abbes refractometer may be calibrated with anyone of the following liquid, Water: 1.3325 Carbon tetrachloride: 1.4969 Toluene:1.4969 a-methylnaphthalene: 1.6176

16. Specific Refraction In 1880, scientist Lorentz showed the property specific refraction which was found to be more useful in characterization of substance independent of temperature . The specific refraction is mathematically expressed as

17. 1 1 S Where, R s is specific refraction of substance n is refractive index of substance p is density of substance

18. Molar refraction It is defined as the product of specific refraction and molecular mass of substance.

19. Where, R is molar refraction of substance n is refractive index of substance M is molecular mass of substance p is density of substance

20. Applications Refractive index values are useful in determination of molecular weights and structures of organic compounds from their molar refraction values . Most commonly it is used to measure the concentration of a solute in an aqueous solution. For a solution of sugar, the refractive index can be used to determine the sugar content.

21. Alcohol content in bioproduction is also determined from the refractometry. Refractive index of a material is the most important property of any optical system that uses refraction for example, lenses and prisms.

22. References Textbook of Physical Pharmaceutics I, Hajare A. Nirali prakashan , 1 st Ed; 2.62-2.66.