Can you explain the phenomenon of wave-particle duality in quantum mechanics, discussing its implications for the nature of particles and waves and how experiments like the Stern-Gerlach experiment and the photoelectric effect have contributed to our understanding?General Relativity: How does the theory of general relativity describe the bending of light around massive objects such as stars and galaxies, and how has this gravitational lensing effect been observed and utilized in astrophysical research, particularly in the study of dark matter and distant objects?