Doing that can reveal a whole bunch of interesting patterns. I was repairing something on a circuit board once and was trying to figure out the wiring. Trying to get more light, I shone my phone’s flashlight at it. At some point I suddenly realized that the circuit board lets a bit of light through. I realized that, at least for some of the board, I could shine a flashlight onto the other side and see pretty exactly how the circuit was laid out. It also looked really, really cool.