Fibre tree
I was at my brothers house the other day and his Christmas tree lights caught my attention. Optical fibres had been used to transfer light from the base to the ends of the branches but what interested me was that the light didn't just come out of the end of the fibres but along the length of the last couple of cm of the fibre. I reckoned that the fibre must be tapered and on closer inspection they were. As soon as I got home I tried simulating the situation with Algodoo, starting with a parallel fibre then cutting the edges to make it tapered.
It looks like my hypothesis was right.