Reflection in physics describes the change in direction of a wave, such as light or sound, when it encounters an interface between two different media. This phenomenon occurs because the wave interacts with the boundaries of the material, causing a portion of the wave's energy to return to the original medium. The fundamental principle governing reflection is the law of reflection, which states that the angle of incidence—the angle at which the incoming wave hits the boundary—is equal to the angle of reflection—the angle at which the returning wave leaves the boundary. The amount of energy reflected relative to the amount transmitted into the second medium is determined by the optical properties and indices of refraction of both materials.