Photonics

Study polarisation and then optical fibres, with practice on intensity, ray geometry, dispersion and link losses.

Advanced Physics
2 lessons

Before you begin

Photonics is the control of light for communication and display. The lessons start with polarisation: polarisers, Malus’s law, the Brewster angle and birefringence. They then use total internal reflection to explain how optical fibres guide light, and what limits how far and how fast a fibre can carry a signal. This topic goes beyond the H3 Physics syllabus.

Be comfortable with: waves and superposition, Snell’s law and the critical angle, and logarithms.

Learning goals

Extend wave optics through polarisation, optical confinement, attenuation, and fibre transport.

Lessons

Work through them in order.

Practise and check

Topic reference

Quick reference

Idea	Relation	Notes
ideal polariser, unpolarised light	I = (1/2)Iᵢₙ
Malus’s law	I = I₀ cos² θ	θ from the light’s polarisation to the analyser’s axis
Brewster angle	tan θB = n₂/n₁	reflected light polarised perpendicular to the plane of incidence
Snell’s law	n₁ sin θ₁ = n₂ sin θ₂
critical angle	sin θc = n₂/n₁	for n₁ > n₂
attenuation	10 log ₁₀Pᵢₙ/Pₒᵤₜ dB	power ratio

Core knowledge to remember

Only transverse waves can be polarised.
At the Brewster angle, the reflected and refracted rays are at right angles.
A birefringent material has different refractive indices for two perpendicular polarisations.
A fibre guides light because the core has a higher refractive index than the cladding.
Dispersion spreads pulses and limits the data rate; attenuation weakens them and limits the distance.
Graded-index fibres reduce modal dispersion, and single-mode fibres remove it.

Common mistakes

Applying Malus’s law to unpolarised light.
Measuring the Malus angle from the first polariser instead of from the light reaching the analyser.
Confusing dispersion with attenuation.
Using 20 log ₁₀ for a power ratio.