The critical angle is the smallest angle of incidence at which total internal reflection occurs when light travels from a denser medium (like water or glass) to a less dense medium (like air).
When light hits the boundary between two media, some of it is refracted (bent) and some is reflected. As the angle of incidence increases, the refracted ray bends further away from the normal. At a certain specific angle—the critical angle—the refracted ray just grazes along the boundary. Beyond this angle, no light escapes; all of it is reflected back into the denser medium. This is total internal reflection.
This principle is crucial in fiber optics, where light signals bounce repeatedly within thin glass strands without escaping, enabling fast and efficient data transmission. It’s also responsible for shimmering effects in diamonds and mirages in the desert.