Cahill's Law is a principle in the field of physics that describes the relationship between the speed of light and the energy of photons. It states that as the energy of a photon increases, its speed approaches the speed of light, but never exceeds it. This law is significant in understanding how light behaves in different mediums and its interaction with matter. The law is named after John Cahill, a physicist who contributed to the study of light and energy. Cahill's Law helps explain various phenomena in optics and is foundational in fields such as quantum mechanics and electromagnetic theory.