An optical fibre has a glass core wrapped in cladding of lower refractive index. Light stays inside because it strikes the core wall above the critical angle and totally internally reflects, zig-zagging along the fibre. If the ray runs too steeply it hits the wall below the critical angle and leaks out into the cladding. Set a shallow enough angle to carry the pulse to the far end.
MissionCarry the light pulse all the way to the far end of the fibre.Round 1Streak 0Best 0
The light stays in the fibre when it hits the wall:
core / cladding index1.50 / 1.40
critical angle69°
angle at the wall75°
at the wall the angle must beat the critical angle for total internal reflection.