Stuck of question D - calculate angle of incidence of this ray at the point of entry to the fibre

Figure 1 shows a side view of a step index optical fibre. Figure 1 light rays in air not drawn to scale B cladding core cladding (b) . Ray A enters the end of the fibre and then undergoes total internal reflection. On Figure 1 complete the path of this ray along the fibre. (c) (i) The speed of light in the core is 2.04 x 10' ms". Show that the refractive index of the core is 1.47. (ii) Show that the critical angle at the boundary between the core and the cladding is about 80. refractive index of the cladding = 1.45 (d) Ray B enters the end of the fibre and refracts along the core-cladding boundary. Calculate the angle of incidence, 0, of this ray at the point of entry to the fibre