Molecules are so very small that they cannot be viewed directly. Nevertheless, each of them has a unique nature and govern macroscopic structure, properties and interactions. The bridge between molecular structure and an observable property can be built when there is a lucid understanding of the structure. The structure of a molecule, though invisible to the naked eye, is not unresponsive to light or electromagnetic waves. Every region of the electromagnetic spectrum has a different effect on molecules. The less energetic microwaves might only rotate molecules, whereas higher energy ultraviolet rays can cause electronic perturbations, or even fragment bonds. Each molecule has one or more unique signatures when they interact with light. The thus transmitted light is detected producing a spectrum. The essence of the course lies in the light-matter interactions and the spectra they produce. From the synthesis of novel compounds or synthesis using novel methods, to quantitative analysis of standard compounds, the development of the chemistry is founded in the ability to characterise chemical compounds.