Both are pretty challenging but completely opposite descriptions of the world in terms of size. Astrophysics deals with the motions, configurations of objects of celestial length scales where the relativistic effects are extremely important. It uses GTR i.e., General theory if relativity, but is essentially classical.

Optics. Optics is the study of light. It is one of the most difficult topics to understand because it requires students to know about waves, especially how they interact with objects and what causes them.