The matter-light interaction in opaque media gives rise to collective effects which may be explained by the coupling between atomic equations and light transfer equation. The trapping of light in an opaque multi-levels atomic system will be studied experimentally in a cold vapour and theoritically. Then, this vapour will be in situation with gain and amplification of light occurs. We will show that a laser threshold exists with this kind of system. Experimentally, the mixing of Raman gain and multiple scattering on a resonant line allowed the abservation a cold-atom random laser. The validity of diffusion equation needs a non frequency shift and so the temperature of atoms should be sufficiently cold to avoid Doppler effect. Finally we study the transport of light in an opaque hot atomic vapour (20°C-180°C). The Doppler effect breaks the Beer-Lambert law for photons step size distribution which is then a Levy flight statictics.
