An optical fibre used for communications has a core of refractive index 1.65 which is surrounded by cladding of refractive index 1.55.
Figure 1
Figure 1 shows a light ray P inside the core of the fibre. The light ray strikes the core cladding boundary at Q at an angle of incidence and angle of refraction of 55.0°.
Calculate the critical angle of the core-cladding boundary.
Calculate the angle of incidence, .
Explain why the light ray enters the cladding at Q and therefore why total internal reflection does not occur.
A different optical fibre is used for the communication. The speed of monochromatic light in the core is 1.64 × 108 m s–1 and the speed in the cladding is 2.51 × 108 m s–1.
Calculate the critical angle for this light at the interference between the core and the cladding.
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