Theory of Refraction:

If you place a pencil in a cup of water, the tip will appear bent. If you then put concentrated salt water in a cup and try the same experiment, the tip of the pencil should appear even more bent. This is an example of the phenomenon of light refraction. Refractometers are measuring instruments in which this phenomenon of light refraction is put to a practical use. Refractometers are based on the principle that as the density of a substance increases (e.g. when salt is dissolved in water) , its refractive index rises proportionately.

Principles of Refractometer:

1.  Refractometers utilise a prism which possesses a much a greater refractive index than the sample solution to be measured. Measurements are made possible using the refractive phenomena which arise at the interface of the prism and the sample solution.

2.  In the case of a weak sample solution, the difference between the refractive index of the solution and that of the prism is great, therefore the angle of refraction is large (see A on the diagram below).

3.  In the case of a strong sample solution, the difference between the refractive index of the prism is smaller and therefore the angle of refraction is smaller (see B on the diagram below).

Calibration Procedure:

 Step 1. 
Flip open the daylight plate on top of the refractometer. Place 2-3 drops of distilled water on the main prism. Close the daylight plate so the sample spreads across the entire surface of the prism.   without air bubbles or dry spots. Never place the refractometer underwater!

Step 2. 
Hold the refractometer (daylight plate) in the direction of a light source and look into the eyepiece. You will see a rectangular field with graduations down the center (you may have to focus the eyepiece to clearly see the graduations by turning it counter-clockwise). The upper portion of the field should be blue, while the lower portion should be white. With the small screwdriver slowly turn the calibration screw until the margin where the blue field meets the white field is at the zero setting.  

Step 3. Using the Refractometer 
Clear the prism of the distilled water. Place 2-3 drops of saltwater on the main prism and look through the eyepiece. Take the reading where the boundary line of the blue and white intersect at the graduated scale. The scale will provide a direct reading of parts per thousand salinity on the right hand scale and specific gravity on the left hand scale. Clean the prism carefully using a damp soft cotton cloth. DO NOT use a paper towel to clean the prism as it may scratch the glass. DO NOT immerse refractometer in water.