What is Refractive Index?

Refractive index plays a major role in controlling the way light propagation in optical fibers. We all experience the effect of refractive index in our daily life. As a child I have wondered by seeing transparent glass or water like drops floating in air when rubbed my eyes and looking outside to the sunlight on a summer day. Later in higher schools, I learned about mirages appearing in the visibility of a traveler in desert. When lived in Saudi Arabia and traveled from Riyadh to Dammam, I could see water on the road.

All the above happens due to the phenomenon of refractive index. Refractive index of a medium (air, water, glass etc) is defined as the relative speed of light in the medium compared to the speed in vacuum. Refractive index is a unit less number as it is a ratio. Refractive index gives us an idea of the medium’s light carrying characteristics. In higher schools, we might have conducted experiment to find out the refractive index of water. The value I got after the first experiment in physics lab was 1.335. Theoretical refractive index value of water is 1.33.

Refractive index (RI) or index of Refraction (IOR), commonly denoted by n in optical communication field, is a dimensionless number. Mathematically, refractive index is the ratio of speed of light in medium divided by the speed of light in vacuum by the equation: n = c/v, where n is the refractive index, c is the speed of light in medium and v is the speed of light in vacuum. A refractive index of 1.46 for a glass material means the light travels 1.46 times faster in vacuum compared to the speed in glass. If the light travels 1.46 kilometers by taking x seconds, it takes x/1.46 seconds only to travel in vacuum. Refractive index of air or vacuum is 1. This is the baseline for our calculations.

Thus, if refractive index of the medium is known, we can calculate the speed of light in the medium. This can be further extended to calculate the time taken for the light to travel a certain distance in the medium. This way, the length of the medium can also be calculated. This principle is used in Optical Time Domain Reflectometer (OTDR).

When light travels from a medium to another medium, both media have different refractive index, the light deflects depending on the refractive index of the medium into which it enters. The angle of such deflection depends on the differences in refractive indices. Suppose light travels in a medium having a refractive index of say x, reaches the boundary of the current medium and enters to another medium having lower refractive index. In this case, the light returns back to the current medium

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