Highlighters and fluorescence

 Thanks to Ben for this brilliant book. I have raced through the first 50 pages and found out a lot of stuff I didn't already know.

The first to grab my attention was highlighter pens.

Now I know that the idea is that ultraviolet photons that we can't see are absorbed, sending electrons up energy levels. The electrons de-excite but don't fall all the way down to the original level in one go. The photons released have less energy and are therefore visible. What I hadn't figured was that the amount of UV in ordinary sunlight coming into my room is enough to release extra photons that make the yellow colour stand out as brighter. Mrs B is knitting with fluorescent yellow for a hi-vis jacket. I was interested to find that you can define a QUANTUM YIELD as the ratio of visible photons released to UV photons absorbed and that 0.1 still counts as fluorescent.