I supervised small groups of undergraduate students taking the PtII (3rd Year) Soft Condensed Matter course, part of the Natural Sciences Tripos at Cambridge. I also taught at the Ogden Trust Senior Physics Challenge - a summer school at the Cavendish Laboratory, organized by Prof. Mark Warner for A-level students considering studying Physics at University. Mark has gone on to develop 'Isaac' a fantastic set of online resources to train A-level students to solve problems and begin to really think like a physicist. Another great resource designed to develop quantitative reasoning is 'Street-fighting mathematics' written by the inspirational Dr Sanjoy Mahajan.
There is a wealth of information on optics and microscopy available online, often actually provided by the suppliers of microscopes and optical components themselves. MicroscopyU is a good place to start as it has some nice interactive tutorials as well as links to review articles. The ThermoFisher website is a good source of information on fluorescence and Semrock has 'Searchlight' a useful interactive tool for selecting fluorophores and filters.To find out more about the physics of light take a look at the optics chapters in Volume 1 of 'The Feynman Lectures on Physics'. Refreshingly, Feynman attempts to explain the behaviour of light in broad conceptual terms, so the lectures make for interesting reading even if you are familiar with the principles of optics. If you're curious about imaging you may be interested in the chapter on geometrical optics - this is the basic approach which informs the design of many optical systems including microscopes. Surprisingly for a physics textbook, there are also chapters on vision! Caltech has generously made all three volumes of 'The Feynman Lectures' freely available to read online.