Why does light converge in a medium?

In order to clarify the concept of gradient force, a transparent dielectric ball is taken as an example. The transparent dielectric sphere is in a Gaussian distributed inhomogeneous convergent light field, and the refractive index of the sphere is greater than that of the surrounding medium. When a focused laser beam irradiates particles, the laser is refracted and reflected, including some absorption. The function of light reflected and absorbed by particles is that the light radiation pressure, or scattering force, is consistent with the direction of light propagation and tends to make the ball move along the direction of light beam propagation. At the same time, the light beam will refract many times after passing through the particle, and the propagation direction of some convergent light rays tends to the optical axis (that is, the propagation direction of the light beam) after refraction, thus increasing the axial momentum, thus giving the particle a force opposite to the propagation direction of light, which is the essence of axial gradient force. Due to this pulling force, the particles can be stabilized near the laser focus in the axial direction. However, due to the inhomogeneity of the light field, the lateral deviation of particles will also be affected by the restoring force pointing to the laser focus, that is, the lateral gradient force. Under the combined action of gradient force and scattering force, particles are stably bound near the laser focus. This is a single beam gradient force optical trap.