It isn’t always easy to wrap your mind around how light works. It’s a particle, but it’s also a wave, and that wave can be powerful. Photons generally travel en masse — we’re talking densities in the range of quadrillions per square inch if they’re coming from the sun. But what if instead of the whole wave crashing over you, you were only hit by a single drop of light — would you know? In other words, is it possible for the human eye to see a single photon?