# Light hitting a GRIN (Gradient-index) lens at the optical angle

In the image below, rays of light are hitting the GRIN (Gradient-index) lens at the optical angle and then converge towards the optical axis. Why do they do this? The refractive index is only a function of the distance from the optical axis and the rays of light are all parallel to that. Without a change in refractive index, the rays of light should not change direction (Snell's law).