An initially parallel cylindrical beam travels in a medium of refractive index μ(I) = μ₀ + μ₂I, where μ₀ and μ₂ are positive constants and I is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius. The speed of light in the medium is:

Refractive index of the medium μ(I) = μ₀ + μ₂I where I = kr², where r = radius of the beam. ⇒ μ(I) = μ₀ + μ₂kr²
Speed of light in the medium v = c/μ(I) where c is the speed of light in vacuum
⇒ v = c/(μ₀ + μ₂kr²)
As r increases, the speed of light in the medium also increases. Hence the speed of light in the medium is minimum on the axis of the beam.