Bright and dark fringes in interference patterns occur due to the phase relationship between coherent light waves. Bright fringes appear where the waves are in phase, reinforcing each other, while dark fringes occur where they are out of phase, canceling each other out. This phenomenon is a fundamental aspect of wave interference, and further details can be found in textbooks and reliable sources like Wikipedia. Understanding these principles is crucial for grasping the behavior of coherent light sources. The interference pattern is a clear demonstration of wave properties in physics.