An initially parallel cylindrical beam travels in a medium of refractive index $\mu (I)={\mu }_0+{\mu }_{2}I$, where ${\mu }_0$ and ${\mu }_2$ are positive constants and $I$ is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius. As the beam enters the medium, it will:

The refractive index $\mu$ depends on intensity $I$. Since intensity is maximum at the axis and decreases with radius, the refractive index $\mu$ is highest at the axis.The speed of light $v=c/\mu$. Thus, the speed of light is minimum at the axis and increases with radius.According to Huygens' principle, the outer parts of the wavefront travel faster than the central part, causing the wavefront to curve inward. This leads to the convergence of the beam.