Figure 33-32 shows four long horizontal layers A–D of different materials, with air above and below them. The index of refraction of each material is given. Rays of light are sent into the left end of each layer as shown. In which layer is there the possibility of totally trapping the light in that layer so that, after many reflections, all the light reaches the right end of the layer?

Figure 33-32 showing four horizontal layers A-D of different materials with their refractive indexes, with air above and below them is given.

Using total internal reflection, a light source can be trapped with a medium. For a total reflection case, the refractive index of the medium having the light source needs to be greater than the refractive index of the medium outside or at the layer of the medium.

• A wave encountering a boundary across which the index of refraction decreases will get rapped totally means undergo total internal reflection.
• So, the adjacent layers should have less refractive index than the layer in which light is incident to get it trapped totally.
• In this case, this condition is satisfied by only layer B as both the materials at the sides of material B have refractive index lower than refractive index of B.
• Therefore, the layer in which there is possibility of totally trapping the light so that, after many reflections, all the light reaches the right end of the layer is layer B.