Optical fibres will be limited to step index only.

The idea

A step-index optical fibre has a core of higher refractive index surrounded by a cladding of lower refractive index, producing a sharp jump in index at the boundary. Light entering within the acceptance angle strikes the core–cladding boundary above the critical angle and undergoes total internal reflection, so it zig-zags along the core with little loss. The specification limits fibre study to this step-index type, so you should not analyse graded-index fibres, where the index changes gradually and rays curve. You should be able to sketch the path, identify core, cladding and buffer, and explain how the critical angle and acceptance angle follow from refractive indices.

What earns the marks

• A step-index fibre consists of a core of higher refractive index surrounded by cladding of lower refractive index, with an abrupt change of index at the core–cladding boundary.
• Total internal reflection occurs at the core–cladding boundary when light meets it at an angle of incidence greater than the critical angle, so light is repeatedly reflected along the core.
• The critical angle is related to the refractive indices by sin C = n₂ / n₁, where n₁ is the core index and n₂ is the cladding index, and n₁ must exceed n₂ for guidance to be possible.
• The acceptance angle at the entrance face determines whether an incoming ray can be guided; rays entering too steeply are refracted out through the cladding rather than trapped.
• Cladding protects the core surface, reduces scattering losses and prevents light crossing between adjacent fibres, while the buffer provides mechanical protection.
• Only step-index behaviour is required: the index changes sharply at one boundary, unlike graded-index fibres where the index varies continuously and rays follow curved paths.

Common mistakes and how to put them right

• Thinking total internal reflection happens at the outer surface of the cladding rather than at the core–cladding boundary.

  Put it right by stating that reflection occurs where the refractive index changes, at the core–cladding interface.

• Believing any ray entering the fibre will be trapped.

  Put it right by explaining that the ray must strike the boundary above the critical angle, which requires entry within the acceptance angle.

• Confusing step-index with graded-index behaviour by describing a gradual change of refractive index.

  Put it right by stating that in a step-index fibre the core index is uniform and changes abruptly at the cladding.

Exam tips

• Draw a labelled longitudinal section showing core, cladding, buffer, the normal at the boundary and the zig-zag ray path, marking angles of incidence and reflection.
• When calculating the critical angle, identify clearly which refractive index is n₁ and which is n₂ before substituting into sin C = n₂ / n₁.
• Link the refractive index difference to the size of the critical angle: a larger difference gives a smaller critical angle and traps a wider range of rays.