Home / Advanced Physics / Photonics Photonics Advanced Physics photonics: study polarisation, then optical fibres, with practice on intensity, ray geometry, dispersion and link losses.
Learning goals Extend wave optics through polarisation, optical confinement, attenuation, and fibre transport. Use polarisation to control transmitted light, then apply wave optics to fibre transmission. These enrichment lessons build on H2 wave optics.
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Quick Reference
Snell’s law: n₁ sin θ₁ = n₂ sin θ₂ Critical angle: sin θ_c = n₂/n₁ n₁ > n₂ Malus’ law: I = I₀ cos² θ I₀ θ Birefringence: Δ n = nₑ-nₒ Attenuation (power, dB): A = 10 log ₁₀(Pᵢₙ/Pₒᵤₜ) Problem Templates
Refraction/TIR: draw the normal, apply Snell, then test against θ_c Polarisers: apply Malus’ law sequentially.Attenuation: use power ratio consistently and avoid amplitude formulas unless the question explicitly uses amplitudes.
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Unpolarised light of intensity $I_0$ passes through a polariser and then an analyser whose transmission axis is at $45^\circ$ to the polariser's. What intensity emerges from the analyser?
$I_0/4$ $I_0/2$ $I_0/8$ $I_0\cos45^\circ$ For light to be guided along a step-index optical fibre by total internal reflection, which relationship between the core and cladding refractive indices is required?
The core index must be greater than the cladding index. The cladding index must be greater than the core index. The two indices must be exactly equal. The cladding must be metallic so that it reflects like a mirror.
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Plane-polarised light of intensity $I_0$ reaches an analyser whose transmission axis is at $60^\circ$ to the incident polarisation direction. What intensity emerges?
$I_0$. $I_0/2$. $I_0/4$. $3I_0/4$. Which combination is required for ray-model confinement by total internal reflection in a step-index optical fibre?
The core refractive index is greater than the cladding index, and the core-boundary incidence angle exceeds the critical angle. The core refractive index is less than the cladding index, and the incidence angle exceeds the critical angle. The core and cladding have equal refractive indices, and the incidence angle is arbitrary. The core refractive index is greater than the cladding index, and the incidence angle is below the critical angle. Unpolarised light of intensity 80 W m⁻² passes through three ideal polarisers. Their transmission axes, measured from the same reference, are 0°, 30° and 90°, in that order. Calculate the intensity after each element. Neglect absorption beyond the ideal polarisation filtering.
Intensity after the first polariser / W m⁻² Intensity after the second polariser / W m⁻² Intensity after the third polariser / W m⁻² Unpolarised light travels in water (refractive index 1.33) towards a flat boundary with glass (index 1.50). The media are transparent, isotropic and nonmagnetic. The reflected beam is completely linearly polarised. Give the incident and refracted ray angles measured from the normal, and the angle between the outgoing reflected and refracted rays. Use degrees.
Incident angle to the normal / degrees Refracted angle to the normal / degrees Angle between the outgoing rays / degrees The same short pulse is sent through three 2 km fibre links at one wavelength. A detector records the following earliest and latest arrival times (µs):
| Fibre | Earliest / µs | Latest / µs |
| --- | --- | --- |
| Step-index multimode | 9.800 | 9.848 |
| Graded-index multimode | 9.800 | 9.812 |
| Single-mode | 9.800 | 9.803 |
Find each measured arrival-time spread in ns, then the factor by which the graded-index link reduces that spread compared with the step-index link. Source and detector contributions are negligible for these measurements.
Step-index arrival-time spread / ns Graded-index arrival-time spread / ns Single-mode arrival-time spread / ns Step-index spread divided by graded-index spread A fibre transmitter launches 2.0 mW. Its receiver needs at least 20 µW. At the operating wavelength, fibre loss is 0.40 dB km⁻¹. Two connectors each lose 0.60 dB, and six splices each lose 0.30 dB. Neglect other losses and engineering margin. Calculate the total permitted loss, the fixed connector-plus-splice loss, and the maximum fibre length.
Total permitted power loss / dB Combined connector and splice loss / dB Maximum fibre length / km
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Light in air reflects from a glass surface ($n=1.50$). At what angle of incidence is the reflected light completely plane-polarised?
$56^\circ$ $42^\circ$ $34^\circ$ $90^\circ$
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Practise and check Course and syllabus information Course Advanced Physics Edition Advanced Physics