The exact point at which TIR occurs is determined by the angle of incidence.
A laser can be used effectively to show how light travelling through different media can totally internally reflect. In Fig. 14.11, the ray is entering air (rare) from water (dense) at an angle greater than the critical angle.
FIG. 14.11 This laser shows how light travelling through different media can totally internally reflect
To demonstrate total internal reflection
1. Set up a ray box to shine a narrow beam of light into a hemi-cylindrical glass block, as shown in Fig. 14.12.
2. Before taking any measurements, note how the light does not bend when it is incident on the block at 90° (i = 0°).
3. Place the ray box such that the light enters the block at three different angles, increasing i each time. Note the measurement of i when the refracted ray runs along the straight side of the block. This is the critical angle, C.
4. Once i exceeds C, total internal reflection should occur, with i = r as in the law of reflection. (In this case, r refers to the reflected angle as the light ray is now undergoing TIR.)
r i incident ray ray box FIG. 14.12 glass block NOTE
If you allow the light ray to enter the block at an angle of incidence of 0°, then no refraction occurs until the ray attempts to exit the glass.
