29 Oct

Index of Refraction

Refractive Index, n, is the ratio of the speed of light in a vacuum and the speed of light in a material.  Refractive index is commonly used as a measure of density for solids and liquids.  The most common use is the determination of concentration of species in aqueous solutions.  A brix refractometer measures the amount of sugar in fruit juices, wine, and other foods and beverages.  The refractometer reads the refractive index of the sample and displays the results as degrees Brix or °Bx.  A saline refractometer measure the amount of NaCl in a samlpe and displays the results in % salt or D=degrees Baume’, Bé°.  Refractometers are found for dozens of measurements including Calcium Chloride, Citric Acid, Copper Sulfate, Glucose, high fructose corn syrup, and Ethyl Alcohol to name just a few.

Refractive index, n, is also very important in optics.  It is refractive index which gives us the ability to create lenses for telescopes and microscopes and cameras.  The fact that light takes a different path through mediums of different refractive index makes it possible to bend and focus light.  Refractive index becomes very important at the highest magnifications in microscopy.  The difference in n between the glass coverslip, the air, and the objective lens makes focusing at high magnifications impossible.  In this case immersion oil is used as a bridge between the cover glass and the objective lens.  the immersion oil has a refractive index which matches the glass so the light is not bent and the objective lens can focus the image.  Refractive index is different at different wavelengths of light.  In the case of lenses refractive index is usually nD the refractive index at the wavelength of the D line of Sodium, 589.592nm which is near the center of the visible spectrum.


Robin Prymula