If the angle of incidence is increased beyond the critical angle, the ray of light does not enter the second (less dense) medium at all. Instead it is reflected back into the first (denser) medium, obeying the laws of reflection as it does. This phenomena is called total internal reflection.
r1 i1 i2 Angle of refraction = 90° r2 C
Total internal reflection occurs Critical angle
Fig 13.8: Light rays strike at various angles, showing critical angle and total internal reflection
Relationship between Critical Angle and Refractive Index
RELATIONSHIP BETWEEN CRITICAL ANGLE AND REFRACTIVE INDEX
n refractive index, C critical angle n 1
sinC
Explanation The critical angle is the angle of incidence when the angle of refraction equals 90°.
The refractive index from the medium to air is , which is equal to . In the case of total internal reflection, i C and r 90°.This implies
mna
mna = sin C sin 90° Q anm = 1
In summary REFRACTIVE INDEX
refractive index (n) sin i sin r real depth
apparent depth 1 DEMONSTRATION 13.1
Demonstration of the Total Internal Refraction and Measurement of Critical Angle
