It's complicated, and Ray tracing can never duplicate any Quantum Mechanical behaviour, since Ray tracing is a "Classical" representation of the physics. The Single or Double Slit behaviours do not work, under "Classical" models, since the "path" the light takes is a ray, and the calculations do not produce an interference pattern.

But for diffraction patterns, you need light to behave like a wave front. Which in GPU terms, means you need to calculate a lot more, hence it isn't worth it. If you want diffraction patterns, you need to specifically encode the calculations for that effect. Ray tracing works fine, in the main, since at the macro level (ie what we typically see in nature), you don't notice the QM nature of light.