Electrical Measurement and Instrumentation

Topics: Total internal reflection, Refractive index, Optics Pages: 6 (1707 words) Published: April 6, 2013
Refractive index measurement is actually a measurement of the speed of light in a medium. The speed of light (usually denoted by c) is 299 792 458 m/s in vacuum. In other media the speed of light is lower than this value, and the refractive index, R.I., of a medium is a measure of how much the speed of light is reduced in the medium. The refractive index (n) of a medium is defined as the ratio of speed of light in vacuum (c) in to that in the medium (v) The speed of light in a medium depends on the medium itself, temperature and wavelength. Due to the wavelength dependency, the refractive index is measured with monochromatic light. The refractive index is commonly determined as part of the characterization of liquid samples, in much the same way that melting points are routinely obtained to characterize solid compounds. It is also commonly used to: * Help identify or confirm the identity of a sample by comparing its refractive index to known values. * Assess the purity of a sample by comparing its refractive index to the value for the pure substance. * Determine the concentration of a solute in a solution by comparing the solution's refractive index to a standard curve. To know about refractive index measurement first of all we know something about refractive index.

What is refractive index?
In optics the refractive index or index of refraction n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium. It is defined as ,

where c is the speed of light in vacuum 
v is the speed of light in the substance
example, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in vacuum as it does in water The historically first occurrence of the refractive index was in Snell's law of refraction, n1sinθ1= n2sinθ2, where θ1 and θ2 are the angles of incidence of a ray crossing the interface between two media with refractive indicesn1 and n2.

Incident ray:- In the diagram, the ray of light approaching the surface of medium is known as the incident ray . Refracted ray:- The ray of light that leaves the surface of medium is known as the reflected ray  Critical angle:- the angle of incidence above which total internal reflection occurs. It is also  defined as tthe angle of incidence that provides an angle of refraction of 90-degrees Example:-

 For the water-air boundary, the critical angle is 48.6-degrees. For the crown glass-water boundary, the critical angle is 61.0-degrees. ni *• sine() = nr • sine ()
ni • sine() = nr • sine (90 degrees)
ni • sine() = nr
sine() = nr/ni
 = sine-1 (nr/ni) = invsine (nr/ni)

Total internal reflection:- Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical anglewith respect to the normal to the surface. Some important features of refractive index

1. Refractive index of materials varies with the wavelength. This is called dispersion; it causes the splitting of white light in prisms and rainbows, and chromatic aberration in lenses 2. The refractive index of a material is the most important property of any optical system that uses refraction. 3. It is used to calculate the focusing power of lenses, and the dispersive power of prisms. 4. It can also be used as a useful tool to differentiate between different types of gemstone 5. It is a fundamental physical property of a substance, it is often used to identify a particular substance, confirm its purity, or measure its concentration 6. commonly it is used to measure the concentration of a solute in an aqueous solution principle of refractive index measurement

Huygens’ principle
When studying the wave nature of light, Dutch physicist Christiaan Huygens developed the theory of wavefronts. When a wavefront of parallel light rays enters the interface between an optically denser material ( A) and an...
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