\f(d) The critical angle is 390 at the interface from plastic to air. (e) Draw a reflection ray and refraction ray at point P. Extend the refraction ray until it reaches the right-side plastic/air interface of the plastic block. (f) Draw surface normal, reflection ray and refraction ray at the incident point on each of the 3 plastic/air interfaces (right, bottom, left) in your Fig. 13. (g) All the angles of refraction and reflection should be drawn to scale using a protractor, and label them in your Fig. 13. Air 20' IP Plastic 11 cm block K . .. . . . .-. Air K.-...-.-9.0 cm .-.=> Figure 13 Rectangle plastic block3. Total Internal Reflection When light passes from a medium of large refractive index into one of small refractive index --- for example, from water to air --- the refracted ray bends away from the surface normal, as shown in Fig. 2. As the angle of incidence increases, the angle of refraction also increases. When the angle of incidence reaches a certain value, called the critical angle, Oc, the angle of refraction is 90. When the angle of the incidence exceeds the critical angle, there is no refracted light. All incident light is reflected back into the medium from which it came, a phenomenon called total internal reflection. Total internal reflection occurs only when light travels from higher-index medium toward lower-index medium. It does not occur when light propagates in the reverse direction --- for example, from air to water. The critical angle Oc can be determined from the Snell's Law since Ga = 0c and Ob = 90. sin 0 =n, sin 90 no , n >no (3) na na Figure 2 shows that total internal reflection will occur if the two conditions are satisfied: (a) na > nb, ( b ) Baz 0c (4) Air Critical angle Total internal Refactedray reflection n' r =90' n Incident ray i i Water Figure 2 Total internal reflection at water / air interface