This chapter explores the interaction of electromagnetic radiation with matter, particularly in the context of spectroscopy. It covers the principles of spectroscopy using ultraviolet, visible, and in...This chapter explores the interaction of electromagnetic radiation with matter, particularly in the context of spectroscopy. It covers the principles of spectroscopy using ultraviolet, visible, and infrared radiation. The chapter explains the wave and particle nature of electromagnetic radiation, highlighting its fundamental properties and explaining how matter absorbs or emits photons.