Huygens' principle is a fundamental concept in wave theory, proposed by the Dutch scientist Christiaan Huygens in the 17th century. It states that every point on a wavefront can be considered a source of secondary wavelets. These wavelets spread out in all directions at the same speed as the original wave, and the new wavefront is formed by the envelope of these wavelets. This principle helps explain various phenomena, such as refraction, diffraction, and interference of waves. By understanding how waves propagate, scientists can better analyze and predict the behavior of light and sound in different environments.