Theory
The refractive index is the ratio of the sine of the angle of incidence of a ray of light in vacuum to the sine of the angle of refraction in the medium, which is equal to the ratio of the wave velocity of light in vacuum to the wave velocity in the medium. The angle are measured relative to the normal of the interface between the two media. For practical purposes, one usually uses air (at 1 atm) as reference medium and not vacuum. The refractive index of air is given with 1.00027.
The refractive index is a physical property like a meting point, a boiling point, or density of a compound. It is very sensitive to the purity of the sample. It depends on the wavelength of the light it is acquired at and the temperature. Most instruments use the yellow sodium line (l=589.3 nm) for the measurement, which is indicated later on as a subscript "D". The temperature has a major effect on the actual reading as can be seen in the formula below:
nD20 = nDobserved + (T-20)*0.00045
Generally, the refractive index decreases with increasing temperature. The medium is less dense, hence the light is less refracted. Most refractive indices in the literature are reported at 20 C or 25 C.
The actual measurement is based on the determination of the critical angle of reflection. The critical angle is defined as the angle at which the angle of refraction is 90o. Any incident angle larger than the critical angle will be totally reflected (at the same angle as the incident angle).
Measurement
The pictures above show the actual refractometer that will be used in the experiment.
1. Applying your sample
Top View
Pull the lever connected to the light downwards as shown in the picture. Then swing the prism open to the left (make sure not to scratch it!). At this point it would be wise to double check the area on which the sample is applied to in order to ensure that it is clean (If not, use a Kim wipe and acetone to clean it. Allow the acetone to evaporate away). On the window that has been exposed upon swinging the prism open (the window directly in the center of the picture on the right), apply the sample. Make sure the sample covers the ENTIRE window with a thin film. Partial application will result in an incorrect refractometer reading and difficulties to observe an 'edge'.
2. Measuring the refractive index
Close the hinged prism and return the light source to its original position (all the way down!).
Locate the power switch on the right lower side in the front of the instrument. Turn the dial to nD (middle). Place the lamp in the back on top of the prism. Now locate the eyepiece at the top of the instrument. You should see crosshairs with a shadow covering the lower part of the viewing range.
