The refractive index in Graded Index (GI) fiber varies radially.

Explanation:
Graded Index fiber is a type of optical fiber that has a varying refractive index along its radial direction. Unlike Step Index fiber, which has a constant refractive index throughout its core, Graded Index fiber has a refractive index that gradually changes from the center to the outer edge.

The refractive index of a material determines how light propagates through it. When light travels from one medium to another with a different refractive index, it bends or refracts. In an optical fiber, the refractive index of the core is higher than that of the cladding, which allows the light to be guided along the fiber.

In Graded Index fiber, the refractive index is highest at the center and decreases gradually towards the outer edge. This variation in refractive index helps in reducing the modal dispersion that occurs in Step Index fiber. Modal dispersion is the spreading of light pulses as they travel through the fiber, which can result in a decrease in signal quality.

The radial variation of refractive index in Graded Index fiber is achieved by doping the core material with different concentrations of dopants. The dopants are chosen in such a way that their refractive index is higher near the center and decreases towards the outer edge of the core. This radial variation creates a refractive index profile that enables the light to follow different paths through the fiber, reducing the modal dispersion.

The radial variation of refractive index in Graded Index fiber allows for the efficient propagation of light signals with reduced dispersion. This makes Graded Index fiber suitable for high-speed data transmission applications, where minimizing signal distortion is crucial.

In conclusion, the refractive index in Graded Index fiber varies radially, with the highest index at the center and decreasing towards the outer edge. This radial variation helps in reducing modal dispersion and improving the performance of the fiber for high-speed data transmission.