Bellingham
& Stanley LtdBellingham
& Stanley Ltd
The fundamental definition of refractive index is based on the speed of light. Light travels at a constant speed in vacuum (approximately 300.000 km/sec) but the speed is reduced when the light passes through any other medium. The ratio of these two speeds is the refractive index of the medium.
The following relationship defines:
Refractive index in a given substance = Speed of light in vacuum / Speed of light in substance
The speed in vacuum is almost identical to the speed of light in air, and since most optical work is carried out in a normal atmosphere it is usual to express the refractive index relative to air rather than vacuum.
A consequence of the change in velocity in different substances is that when a ray of light passes obliquely from one substance to another there is a change in direction of the ray. This deviation is called refraction. The refractometer measures the deviation suffered by a light ray under defined and controlled conditions of illuminations.
In essence, the refractometer compares the unknown refractive index of the sample against the known refractive index of a prism and by optical laws enables the index of the sample to be read direct from a graduated scale.
The most common, commercial! use of refractometers is in quality control in the food industry brought about by the phenomena that when a soluble substance such as sugar is dissolved in water the refractive index of the solution changes as a function of the concentration. This applies to all soluble solids in varying degrees, however, since sugar is one of the most widely used substance in the food and beverage industries, the majority of instruments made for such applications, are calibrated directly in terms of sugar percent and may or may not have a related refractive index scale.
Bellingham & Stanley Ltd have been engaged in the design and manufacture of refractometers for a very wide range of applications since its formation in 1914. The company specialised in such instrumentation and as such, is probably the major manufacturer in the word, dedicated to this particular sphere. Its products are sold all over the world principally in to the food, petroleum and pharmaceutical industries. It carries out its own research and development for new requirements and continual updating of the design of its product range.