One ray represents many photons, and those photons scatter off in many different directions when they hit most surfaces. To get a reasonable picture of where th

One ray represents many photons, and those photons scatter off in many different directions when they hit most surfaces. To get a reasonable picture of where they went, you need a lot of samples. A single raycast also only gets you one step of the light's journey. That means, for example, that following light as it bounces from a light source, to a surface, to another surface to your eye requires at least 3 rays even for perfectly mirrored surfaces.