Snell’s Law explains total internal reflection by describing how light behaves when it moves between two media with different refractive indices. When light travels from a denser medium (higher refractive index) to a rarer medium (lower refractive index), it bends away from the normal. As the angle of incidence increases, the angle of refraction also increases.
At a specific angle called the critical angle, the refracted ray skims along the boundary between the two media. If the angle of incidence increases beyond this critical angle, Snell’s Law predicts that no real angle of refraction exists—the sine of the refraction angle would be greater than 1, which is not possible. Instead of refracting, the light is completely reflected back into the denser medium.
This is called total internal reflection, and Snell’s Law helps calculate the critical angle at which this happens. The phenomenon only occurs when:
- Light moves from a higher to a lower refractive index.
- The angle of incidence is greater than the critical angle.
Thus, Snell’s Law provides the foundation for understanding when and why total internal reflection takes place.