2. The Critical Angle/Total Internal Reflection critical angle: the angle at which light is refracted at 90º. total internal reflection: the reflection of incident light from a boundary due to the incident light angle exceeding the critical angle. The critical angle and total internal reflection are applicable only when light travels from a medium with a greater index of refraction to a medium with a smaller index of refraction (n 1 >n 2 ).

3. θ2θ2 θ1θ1 θ1θ1 θ2θ2 n1n1 n2n2 Refraction Critical AngleTotal Internal Reflection θ2θ2 θcθc The incident light ray is less than the critical angle (θ 1 <θ c ). The incident light is at an angle that causes the transmitted light to refract at 90º. (θ 1 =θ c, θ 2 =90º). Any incident angle greater than the critical angle causes light to be reflected from the the boundary between the two media. (θ 1 >θ c, θ 1 =θ 2 ) Light is transmitted to the new medium with a new angle. The transmitted light is refracted at 90º. The incident light is reflected from from the boundary. The law of reflection applies. n1n1 n1n1 n2n2 n2n2

4. When the transmitted light refracts at 90º, θ 1 =θ c and θ 2 =90º n 1 sinθ c =n 2 sin 90º n 1 sinθ c =n 2 n 1 sinθ 1 =n 2 sinθ 2 Any incident angle greater that the critical is reflected from the boundary of the two media. The index of medium 1 must be greater than the index of refraction of medium 2 for the critical angle and total internal reflection to occur.