A beginners guide to understanding the mechanisms of photochemical reactions: things you should know if light is one of your reagents.

The increasing popularity of applied photochemistry has changed the composition of the practitioners of photochemistry, from traditional specialists, to users whose expertise lies elsewhere, yet they find light as a useful and powerful reagent. I introduce Kasha's rule very early in this tutorial; this unconventional approach allows me to bypass information about high electronic states in favor of the lowest singlet and triplet excited states. Doing this I try to provide a fast entry enabling newcomers in the field of applied photochemistry to have a taste of what the field has to offer, in the hope that they will like what they see, and venture further into the many resources available to go deeper into the fascinating field of organic photochemistry.