The double-slit experiment is a famous physics experiment that demonstrates the wave-particle duality of light and matter. In the experiment, a beam of particles, such as electrons or photons, is directed at a barrier with two closely spaced slits. When both slits are open, an interference pattern appears on a screen behind the barrier, indicating that the particles behave like waves. When one slit is observed or measured, the interference pattern disappears, and the particles behave like individual particles. This surprising result suggests that the act of measurement affects the behavior of particles, raising questions about the nature of reality and observation in quantum mechanics.