Fermat's Principle, also known as the principle of least time, states that light travels between two points along the path that takes the least time. This principle was formulated by the French mathematician Pierre de Fermat in the 17th century. It helps explain various optical phenomena, such as reflection and refraction, by showing how light behaves in different media. According to this principle, when light encounters a boundary between two materials, it will change direction to minimize travel time. This behavior can be observed in everyday situations, like how a straw appears bent when placed in a glass of water, illustrating the effects of light changing speed in different substances.