Paul Langevin was a French physicist born on January 23, 1872, and is best known for his work in the fields of ultrasonics and theory of relativity. He made significant contributions to the understanding of wave phenomena and developed the concept of ultrasonic waves, which are sound waves at frequencies above the audible range for humans. Langevin also played a crucial role in the development of sonar technology during World War I, which helped detect submarines. His research laid the groundwork for various applications in medical imaging and industrial testing, making him a prominent figure in 20th-century physics.