The refractive index of a medium with reference to air is equal to the ratio of the sine of the angle of incidence of a beam of light in air to the sine of the angle of refraction of the refracted beam in the given medium.
- Unless otherwise prescribed, the refractive index is measured at 20 ± 0.5 °C, with reference to the wavelength of the D-line of sodium (λ = 589.3 nm);
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- Wipe down the upper surface of the main prism before dripping contact liquid onto it.
- Put a tiny quantity of contact liquid on the center of the prism in the case of a glossy and flat glass sample, put a tiny quantity of contact liquid on the center of the glass sample, put a tiny quantity of contact liquid on the center of the glass surface to contact the prism. One drop of 1 mm in diameter is a proper quantity.
- When white light is used, the Refractometer is provided with a compensating system. The apparatus gives readings accurate to at least the third decimal place and is provided with a means of operation at the temperature prescribed. The thermometer is graduated at intervals of 0.5 °C or less.
- The optical parts of the apparatus should be kept brilliantly clean.
- The working surfaces of prisms should be free from scratches.
- The refractive index of distilled water, which is 1.3330 at 20 °C and 1.3325 at 25 °C.
