 The speed of light in a vacuum is always the same.  but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material.  The ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction for the substance.

This is an example of the phenomenon of light refraction.

1-Refractometer is using a prism which has a much greater refractive index than the sample solution to be measured. Measurements are made possible using the refractive phenomena which arise at the surface between 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).

 In most liquids and solids the speed of light, and hence the index of refraction, varies significantly with wavelength.  (This variation is referred to as dispersion, and it is what causes white light moving through a prism to be refracted into a rainbow. Shorter wavelengths are normally refracted more than longer ones.)

 Light (1) enters through the rear of the refractometer. then hits a mirror (2).  which transmits the light to the center of the hemicylinder (3),  At the boundary between the hemicylinder and the gemstone (4), the light will be partially refracted inside the stone and partially reflected in the hemicylinder.  The reflected rays (5) will pass through a reading scale (6) and a lens (7) or a series of lenses, depending on the type of refractometer.

 The reflected rays hit a mirror (8) which directs the light to the ocular (9) and then outside the refractometer to your eye (11).  The ocular (9) can slide in and out for better focus and is usually accompanied with a detachable polarizing filter (10).

1.Help identify or confirm the identity of a sample by comparing its refractive index to known values. 2.Assess the purity of a sample by comparing its refractive index to the value for the pure substance. 3.Determine the concentration of a solute in a solution by comparing the solution's refractive index to a standard curve. 4.Gemology.