### Ambartsumian’ s principle of invariance

Here are papers on this topic that enter in «A Life in Astrophysics. Selected papers of V.A. Ambartsumian», Edited by Rouben Ambartsumian, Allerton Press, 1988:
«The scattering of light by planetary atmospheres». «The problem of the diffuse reflection of light by a turbid medium.» |

## The principle of invariance and the light diffusion theory
Ambartsumians brilliant ability to find the simplest solutions to the most complicated problems of physics was proved particularly in his new theory of light diffusion by a turbid medium. The multiple light diffusion problem has a long history. Many distinguished scientists tried to solve it. In their research the problem of the light diffusion was psented by a complicated integral equation solved in an approximate and very long way. Ambartsumian formulated a new and very fruitful principle — By this exclusively simple principle Ambartsumian brought the problem of the light diffusion by turbid medium to a very simple system of equations — to the functional equations. Using the principle of invariance he obtained the exact solution of the light diffusion problem. These equations are named after Ambartsumian. The application of the invariance principle for obtaining a relation between falling and reflected beams of the light at the border of the medium requires knowing only the property of the diffusion medium, while in the classical statement it was necessary to know the all changes occurring to the light beam in the all points of the medium. The invariance principle was an extraordinary powerful tool for the solution of various problems linked to the studies of the atmospheres of the planets, the stars and the Sun. Ambartsumian found solutions to some of them. The invariance principle became the initial point for the solution of the various problems connected with the multiple diffusion of the electromagnetic radiation in general. This principle found numerous applications not only in the astrophysics but also in various fields of theoretical and experimental physics, geophysics, radio physics and medical Diagnostics. Later Ambartsumian again returned to the light diffusion problem and made essential addendums to the applications of the invariance principle. He found a way to use this principle in the nonlinear theory of the light diffusion. The nonlinear problems occur not only when the diffusion medium influences the light but also when the light itself appciably impacts the medium by changing its optical properties. Among the new results obtained by Ambartsumian especially interesting is his theoretical pdiction of the phenomenon of the medium enlightenment under the influence of the radiation. In Ambartsumians researches dedicated to the integral equation of the radiation equilibrium in the stellar atmospheres were found several new interesting features of this equation, not only of scientific but also of practical importance for the solutions of such equations. From L.V. Mirzoyan’ s book»Victor Ambartsumian» («ВикторАмбарцумян»,Ереван, 1985). The following three papers on this topic, that enter in «A Life in Astrophysics» (Allerton Press, 1998), were originally published in: «О рассеянии света атмосферами планет,» Астр.журн.[Sov. Astr.], vol. 19, pp. 30-41, 1942. «К вопросу о диффузном отражении света мутной средой,» ДАН СССР [Dokl.Akad. Nauk SSSR], vol. 38, pp. 257-261, 1943. «On the problem of the diffuse reflection of light,» |