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Refractometry

Refractometry. refractive index or n :. The speed of light in a vacuum is always the same. but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material.

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Refractometry

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  1. Refractometry

  2. refractive index or n : • The speed of light in a vacuum is always the same. • but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material. • The ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction for the substance.

  3. This is an example of the phenomenon of light refraction.

  4. Principle: -Refractometer is using a prism which has a much greater refractive index than the sample solution to be measured.

  5. Abbe refractometer: • Abbé refractometer working principle is based on critical angle. • Sample is put between two prisms - measuring and illuminating. • Light enters sample from the illuminating prism, gets refracted at critical angle at the bottom surface of measuring prism, • then the telescope is used to measure position of the border between bright and light areas.

  6. Telescope reverts the image, so the dark area is at the bottom, even if we expect it to be in the upper part of the field of view. • Surface of the illuminating prism is matted, so that the light enters the sample at all possible angles, including those almost parallel to the surface. • To prevent dispersion , Abbé added two compensating Amici prisms into his design. • Not only telescope position can be changed to measure the angle, also position of Amici prisms can be adjusted, to correct the dispersion.

  7. What is dispersion? • In most liquids and solids the speed of light, and hence the index of refraction, varies significantly with wavelength. • (This variation is referred to as dispersion, and it is what causes white light moving through a prism to be refracted into a rainbow. Shorter wavelengths are normally refracted more than longer ones.) • If the light source is not monochromatic , light gets dispersed and shadow boundary is not well defined, instead of seeing sharp edge between white and black.

  8. Application: • Help identify or confirm the identity of a sample by comparing its refractive index to known values. • Assess the purity of a sample by comparing its refractive index to the value for the pure substance. • Determine the concentration of a solute in a solution by comparing the solution's refractive index to a standard curve

  9. R= (n ² - 1 ) x Mwt / (n² + 2 ) d. Where: R: molar of refraction. n: refractive index. d: density. Mwt.:molecular weight.

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