The critical angle in refraction is the minimum angle of incidence at which light, traveling from a denser medium to a rarer medium (like from water to air or glass to air), is totally reflected back into the denser medium rather than refracted into the rarer medium.
Key Points:
- It occurs only when light moves from a higher refractive index to a lower refractive index.
- At the critical angle, the refracted ray travels exactly along the boundary between the two media.
- If the angle of incidence is greater than the critical angle, total internal reflection happens — the light is completely reflected inside the denser medium.
Example:
- For water to air, the critical angle is about 48.6°.
- For glass to air, it is about 42° (varies by glass type).
This principle is used in optical fibers, prisms, and mirage formation, where controlling light reflection is essential.