What are examples of physics topics that are considered to be modern physics?
Areas of physics such as Relativistic Mechanics and Quantum theory
Till the end of the 19th century physics was broadly classified in concentrations (e.g. Classical Mechanics including gravitation, Waves and Optics, Statistics Physics and Electricity and Magnetism).
However, at the end of the nineteenth century phenomena were observed which were not explained on the basis of these theories alone (photoelectric effect, Michelson Morley experimental results, Geiger-Marsden experiment, Atomic and nuclear structures, quantum effects etc.).
With improvement in measurement technology and instrumentation other phenomena also did not agree with predictions of classical physics (wave particle duality, Compton effect , pair production etc.)
This required new theories to account for the deviation between Classical Physics predictions and observations.
Broadly speaking all explanations and theories that take into account the areas not covered in the Classical Physics Concentrations are termed as Modern Physics.
Majority of which are now explained by using Relativistic Mechanics and Quantum theory.