The classical view of the propagation of light regarding superposition, reflection, or refraction, assumes a linear relationship between the electric field and the propagating medium when the light comes from a traditional point source. However, as the intensity of light increases, the relationship between the propagating medium and the beam of light interact in a nonlinear way .
For example, take a 200-megawatt pulse (at the most extreme case) from a Q-switched Ruby laser, propagating through the air would result in an electrical field amplitude of approximately 10⁸ volts-per-meter, when calculating with the linear equation. This is two orders of magnitude greater than the breakdown of air molecules. Assuming that the world of optics behaves in a linear way, any high-powered laser would separate most diatomic molecules in the air. Therefore we must consider that for high intensities of light there is a nonlinear relationship with the propagating medium .
 E. Hecht and A. Ganesan, Optics, 5th ed. (Pearson, 2017), pp. 667-671.