Self-focusing

Self-focusing is a nonlinear optical effect where intense light passing through a medium causes the medium’s refractive index to change, creating a lens-like effect. In other words, the high light intensity makes the medium act like it’s "focusing" the beam itself, causing the light to converge more tightly. This phenomenon often occurs in materials like glass or certain gases at high power levels. Self-focusing can be useful in applications like laser guiding or can lead to undesirable effects such as beam distortion or damage if uncontrolled.