Light (or electromagnetic energy) can be seen both as a wave and a particle. The famous two slit experiment showed us that exactly: when only one slit is present, the pattern behind the slit indicates a particle-like behavior. When two slits are present but only the screen behind them is observed, the screen show evidence of a wave-like interference pattern. If a detector is then added to one of the two slits, the pattern on the back screen again shows a particle-like behavior, as if all particles have gone through the other slit. It is almost as if the particles or waves do not wish to be observed.