# Refractive index

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A refractive index is a unit that is used to measure the speed of light in that specific material. The speed of light in a vacuum Is very much faster than any other medium. The speed of light in a vacuum is approximately 300,000 kilometers (186,000 miles) per second. We can simply calculate the refractive index of a medium by diving the speed of light in that medium to the speed of light in a vacuum. The refractive index of vacuum is equal to 1. A basic single-mode fiber that is used in telecommunication purposes consists of a cladding that is purely made of silica, which has a refractive index of 1.444 at 1500nm and there is also a core at the center which is made of doped silica with a refractive index of 1.4475.

The speed of light in a medium is inversely proportional to the refractive index of that material, which means the higher the refractive index will be, the lower the speed of light will be. From this information, a simple thumb rule is that a single optical fiber that is used for the purpose of telecommunication is capable to travel at a speed of 200,000 kilometers per second. Thus, a phone call made by fiber over a distance of 16,000 km between Sydney and New York means there is a minimum delay of 80 milliseconds between one caller speaking and the other. Precision optics manufacturers are selected from the best optical lenses manufacturer to manufacture custom lenses and do prism products selection.

### Total Internal reflection:

When a light that s traveling in a medium that is optically dense strikes a boundary at a steep angle (which is larger than the critical angle when measured from the boundary), then the light gets reflected completely. This whole phenomenon is known as Total Internal Reflection. The use of this effect is made in optical fiber for the confinement of light in the core. Most modern optical fibers have weak guidance, which means that the difference in refractive index between the core and the cladding is very small (which is usually less than 1%).

Light travels through the fiber core, and then it bounces back and then forth off the boundary between the cladding and the core. Because the light must strike the boundary with an angle greater than the critical angle, only light entering the fiber within a certain range can travel through the fiber without leaking.