In optics, the critical angle is the greatest angle at which a light ray, travelling in one transparent medium, can strike the boundary between that medium and a second less dense medium without being totally reflected within the denser medium.
As light passes through different substances, its frequency remains constant but its wavelength (speed) changes. This change of wavelength at the surface interface between different mediums causes light passing through one and into the other to be bent, or refracted. If light strikes the boundary between the two mediums at any angle less than the critical angle, and if it strikes the boundary from the other side, a part of the beam will penetrate the boundary, being refracted in the process. However, light striking the boundary at an angle larger than the critical angle undergoes total reflection and cannot pass to the less-dense medium.