The interaction of solar radiation with clouds, greenhouse gases, and aerosol particles impacts the Earth’s energy budget, and hence, its climate. Here, we introduce the basic concepts of solar (or shortwave) and terrestrial (or longwave) radiation and fundamental principles, such as black-body radiation, scattering, the Stefan–Botzmann, and the Kirchhoff law. The concepts are needed to understand how the radiation interacts with objects such as hydrometeors in clouds or aerosol particles. This interaction is called radiative transfer in the atmosphere. With the principles of radiative transfer established, this chapter discusses the energy balance of the Earth–atmosphere system, separately at the top of the atmosphere, the atmosphere and Earth’s surface. A simple layer model is introduced to understand the greenhouse effect. Radiative effects are discussed separately for different cloud types. The chapter concludes by discussing some atmospheric phenomena commonly observed in the atmosphere, such as halos and glories. Rainbow formation, geometric optics and ray tracing are discussed in detail.

We start from the Planck thermodynamic description of light that leads to black-body radiation. We discuss Einstein’s introduction to spontaneous emission and the first evidences leading to the realization of the laser. We detail the nonequilibrium descriptions of lasers as heat engines. We define random lasers and detail their introduction from Letokhov. We show their link with modern optical computing and the Ising and Hopfield models. This chapter illustrates the idea of complexity, the relation with replica symmetry breaking, and the role in optical computing and Ising machines.

Equilibrium distribution functions are determined for fermions (e.g. electrons) and bosons (e.g. photons). The Saha–Boltzmann equation, the Maxwellian distribution, and relativistic Maxwell–Juttner distribution are derived. The relativistic equation of state for a distribution where particle velocities approach the speed of light is examined.

Max Planck was an expert on classical thermodynamics and turned his attention to the understanding of black-body radiation in 1895. He worked out the form the radiation formula had to have from the emission and absorption of radiation of a dipole in thermal equilibrium with the radiation it emits. The formula related the mean energy density of radiation to the average energy of the oscillator. Using new precision measurements of the black-body spectrum, Planck derived the primitive form of the black-body spectrum by a combination of theory and the empirical results of experiment, but without any physical interpretation of the significance of the formula.

The story of the discovery of quantisation and quanta begins with the numerous problems facing physicists at the end of the nineteenth century. A number of experimental results did not fit naturally into the scheme of classical physics. Among the most important of these was the spectrum of black-body radiation, which was being determined much more precisely experimentally in the last decade of the nineteenth century and which had to be explained theoretically. Important clues came from the pioneering studies of the origin of the Stefan-Boltzmann law and Wien's displacement law, the latter involving the clever use of dimensional methods.