Read Optical Fibres: Trace Refraction to the Core Boundary
An optical fibre accepts a ray only if refraction at the entrance still allows total internal reflection at the core-cladding boundary.
Optical-Fibre Acceptance and Total Internal Reflection is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Start with the condition
Use Snell's law at the air-core face, then relate the internal ray angle to the incidence angle at the core-cladding interface. Apply the critical-angle condition there.
Build the governing relation
The largest accepted entrance angle defines the numerical aperture; use n-core greater than n-cladding throughout.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
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- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
Applying the critical-angle condition at the air-core surface instead of the core-cladding surface changes the physical problem.
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