Chapter 4 is concerned with Mandelstam's research on optics conducted in Strasbourg. Critical intentions of young Mandelstam are pointed out. Mandelstam's attack on Rayleigh's classic theory of the blue color of sky along with his polemics with Max Planck concerning the dispersion of light is described. The author specially discusses the one-sided elucidation of this polemics in Soviet scientific literature and he attempts to reconstruct reasons of this narrowness.Mandelstam's creative evolution is traced. In this connection Mandelstam's article about roughness is pointed out and it is noted that this article attracted the attention of a number of famous physicists (including A. Einstein).