That's why the lessons are online. You can read through them and everything is explained there. In ray-tracing light most often travels from the eyes to the sur

That's why the lessons are online. You can read through them and everything is explained there. In ray-tracing light most often travels from the eyes to the surface and from the surface to the lights (direct illumination). Various laws (such as as the inverse square and lambert-cosine laws) are applied to simulate reality.