The Numerical Aperture is a dimensionless quantity that determines the ability of a system to gather light. It is the measure of the range of angles over which the system can accept light.
NA is calculated in terms of the refractive index (n), which is also a dimensionless quantity that describes the change in the velocity of light when traveling through that medium, and the acceptance angle (θ), which is the maximum incident angle at which an optical system will permit light into it.
Speed of light in vacuum, c = 3Χ108 m/s
Acceptance angle in fiber optics is defined purely based on ray optics (geometrical aspect). According to the ray model of light, the angle of incidence of a ray determines whether or not it will be coupled into the fiber core and allowed to transit through it via total internal reflection. The maximum incidence angle of an optical fiber at which the light can enter into the core is known as the acceptance angle. So, any ray with an incident angle less than the acceptance angle can be guided through the fiber. The Acceptance cone is the cone within which an optical ray may be coupled into a fiber core. It is derived by rotating the acceptance angle about the fiber axis.
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