Laboratory Tutorials - Index of Refraction

Background

Mathematically, the index of refraction is the ratio of the speed of light through a vacuum and the speed of light through the medium in question. The refractive index can also be calculated by measuring the angle of incidence and the angle of refraction and applying the formula:

n = sin(θ_i) / sin(θ_r)
(where n is the index of refraction)

The index of refraction is related to the physical structure of the medium through which light is passing. For this reason, the index of refraction is a characteristic of substances that can be employed in identifying unknowns. Index of refraction can be calculated if the structure of a compound is known, or the refractive index can be experimentally determined. To calculate the index of refraction, the sum of all of a compound's component parts will yield the refractive index for the compound. For example, the refractive index of a carbon atom is 2.42 and a double bond contributes 1.73 to the value, then propene, a compound with three carbon atoms and one double band will have a calculated index of refraction of 8.99. If the structure of the compound in question is unknown, the index of refraction can be experimentally determined by measuring the angles of incidence and refraction as light passes through a substance. Then, use the equation:

n²⁰ = n^t + .0004(T - 20)
(where n represents the refractive index)

Determining refractive index using the ABBE Mark II Refractometer

$ABBE Mark II Refractometer$

Place a drop or two of the liquid being tested on the lower crystal.
While squeezing the spring clamp, lower the upper prism so that the liquid is now contained between the two crystals.
Turn the mode selector on the front of the instrument to the ND position.
Raise the lighted arm into position behind the crystals and look through the eyepiece. You should observe a border between light and dark in the view finder. The view may be focused by turning the eyepiece itself. Using the handwheel on the right side of the instrument, adjust this border until it is in the center of the field of vision. If the view appears dark, the adjustment should be in the counterclockwise direction; if the view appears bright, the adjustment should be in the clockwise direction. The actual border line may be strongly colored.
To make the borderline as sharp as possible and to minimize the amount of color that is surrounding the border, adjust the position of the dial that is located underneath the eyepiece.
Bring the borderline directly on the crosshairs of the viewfinder using the control wheels on the right side of the instrument. The small wheel should be used for coarse adjustment; the larger wheel utilized for more fine adjustments. The final adjustment should be in the clockwise direction.
Press the switch on the front left-hand side of the refractometer. The digital display will show the refractive index to the fourth decimal place.
Press the switch on the front right-hand side of the refractometer to measure the temperature of the liquid being tested.
Open the prisms. Blot (DO NOT RUB!) the prisms dry using a clean, dry lens tissue. Clean the lenses using a lens tissue that has been moistened with alcohol.

Applying the experimental refractive index

Refractive index is a physical property that is dependent on the temperature of the liquid that is being tested. Literature values for refractive index, however, will only be given for one particular temperature (usually 20° Celsius). For this reason, it is necessary to be able to manipulate the experimental values of refractive index so that they can be compared to the values found in literature.

The equation that governs this relationship is:

n²⁰ = .0004(T - 20)

Knowing this relationship, and the temperature at which the refractive index was experimentally determined, the experimental value can be transformed in such a way that it can be properly compared to the literature value.