Paradoxes of Thermal Radiation

This paper presents an analysis of the thermal behaviour of objects exposed to a solar-type flux of thermal radiation. It aims to clarify certain apparent inconsistencies between theory and observation, and to give a detailed exposition of some critical points that physics textbooks usually treat in an insufficient or incorrect way. In particular, the paper examines the equilibrium temperature reached by objects exposed to solar thermal radiation and the temperature difference between their illuminated and shaded sides. These problems are studied first by analyzing the simple ideal case of an isolated object, subsequently by taking into account the thermal radiation emitted by the environment, and finally by considering also the heat exchange with the surrounding air. Some examples are developed and numerical data are provided. The topic is developed in a way which can be suitable both for undergraduate student and general physicist.