Optical/IR interferometry: history and basic principles

The present notes refer to a series of lectures delivered in September 2017 in Roscoff (France) during the 2017 Evry Schatzman School, at IUCAA (Inter-University Center for Astronomy and Astrophysics, Pune, India) in April 2018, during the July-September 2020 period and during the November 2020-January 2021 period via on-line lectures organized by IUCAA and by the Astronomical Observatory Institute, Adam Mickiewicz University in Poznan, respectively.
It concerns a general introduction to optical/IR interferometry, including a brief history, a presentation of the basic principles, some important theorems and relevant applications. The layout of these lecture notes is as follows. After a short introduction, we proceed with some reminders concerning the representation of a field of electromagnetic radiation. We then present a short history of interferometry, from the first experiment of Fizeau and Stefan to modern optical interferometers. A first intuitive approach of the Zernicke-van Cittert theorem is proposed. We then discuss the notions of light coherence, including a rigorous demonstration of the theorem of Zernicke-van Cittert and describe the principle of interferometry using two telescopes. We present some examples of modern interferometers and observational results obtained with these. Finally, we address three important theorems: the fundamental theorem, the convolution theorem and the Wiener-Khinchin theorem which enable to get a better insight into the field of optical/IR interferometry.