The critical angle is the minimum angle of incidence at which light, moving from a medium with a higher refractive index to one with a lower refractive index, is refracted along the boundary between the two media. Beyond this angle, total internal reflection occurs, and no light passes into the second medium—it is completely reflected back into the first.
Snell’s Law helps define the critical angle by relating the angles of incidence and refraction to the refractive indices of the two media. At the critical angle, the angle of refraction becomes 90 degrees, meaning the refracted ray travels exactly along the surface. Using this condition, Snell’s Law allows us to determine the specific angle at which this transition occurs.
The critical angle depends on the refractive index difference between the two materials. A larger difference results in a smaller critical angle, making total internal reflection easier to achieve. This concept is essential in applications like fiber optics, prisms, and optical instruments, where controlling the path of light is crucial.