10: Refractive Index

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The refractive index is a ratio of the speed of light in a medium relative to its speed in a vacuum. This change in speed from one medium to another is what causes light rays to bend. It is also important to note that light changes direction when it travels from one medium to another.

$diagram showing refraction of light$

Refractive indices have many purposes and are used most frequently to differentiate between liquid samples. Therefore, this physical quantity characterizes liquids in the same way that melting points are used to characterize solids. This measurement can serve as a means of identification of a substance by comparing its refractive index to known literature values. Refractive indices can also be used to as an estimate of the purity of a compound by comparing the substance’s refractive index to that of the pure compound.

Two factors affect the value of the refractive index are:

Temperature: Refractive index values are usually determined at standard temperature.
Wavelength of light: The refractive index varies with wavelength. Only a specified wavelength should be used when measuring the refractive index.

A refractometer is used to measure the refractive index of a medium. A light source shines on the illuminating prism and light rays enter the sample moving in different directions. A detector at the back of the refracting prism produces the light and dark regions. Samples with different refractive indexes produce different angles of refraction which will cause a shift in the borderline between the light and dark regions. The borderline’s position is then used to establish the refractive index of different samples by viewing an illuminated scale.

Screenshot (75).png

Refractive Index Lab Procedure

Open prism assembly and inspect the prism for cleanliness. If necessary, clean with acetone and a Kimwipe.
Place a few drops of the liquid sample on the lower prism using a pipette and close the prism.
Move the illuminator arm upwards.
Turn the adjustment control until the lower field appears dark and the upper field light.
Press contact switch at left side of instrument. Reading the top scale, estimating to the 4^th decimal place.
Take a temperature reading from the attached thermometer.
Clean the prisms by wiping the sample with a lens tissue, followed by cleaning with acetone.

Temperature Correction

Since refractive indices vary with temperature, for each degree Celsius of temperature change, the average temperature correction has been found to be 0.00045 units for a wide range of compounds. The following equation can be used to determine the refractive index at 20.0ºC:

Screenshot (76).png

Ex. If the reading for the refractive index was 1.4370 at 18ºC, it would be corrected to 1.4370- 0.0009 = 1.4361 at 20ºC.

Refractive indices are reported in literature at 20.0ºC. In order for comparisons to be made between the literature and experimental refractive index, the experimental refractive index must be corrected to 20ºC.

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