Modelling Young's double slit experiment
It is in fact possible to augment a ray tracer to make it capable of simulating wave effects. The paper Rendering Wave Effects with Augmented Light Field
 describes one way to do it. In summary, they introduce a framework called Augmented Light Field that allows them to model wave effects with a ray-based representation. In this framework, rays can carry negative radiance in addition to positive radiance; intuitively, rays carrying negative radiance can "subtract" light from surfaces that it cannot reach because of interference effects, for example.

I haven't implemented it personally, and I can't speak for its implementation complexity, but I believe it is a good starting point.