The document discusses several topics in atomic and nuclear physics: 1) Light exhibits properties of both waves and particles, known as wave-particle duality. Photons and the photoelectric effect show light acting as a particle, while experiments like interference and diffraction show its wavelike properties. 2) The Compton effect demonstrates that X-rays can lose energy when scattered by electrons, indicating X-rays act as particles. X-rays are also produced when high-energy electrons interact with atoms. 3) Atoms have distinct energy levels, and photons are emitted or absorbed when electrons transition between these levels. The energy of photons equals the energy difference of the levels.