The critical angle is the specific angle of incidence at which light traveling from a denser medium to a less dense medium is refracted at an angle of 90 degrees. Beyond this angle, total internal reflection occurs, meaning that all the light is reflected back into the denser medium instead of passing through. This phenomenon is crucial in optics and is commonly observed in materials like glass and water. The critical angle can be calculated using Snell's Law, which relates the angles of incidence and refraction to the indices of refraction of the two media. The formula is given by: sin(θc) = n2/n1, where θc is the critical angle, n1 is the refractive index of the denser medium, and n2 is that of the less dense medium. Understanding the critical angle is essential in designing optical devices such as fiber optics and lenses.