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GCSE Physics · key term
The straight reference line through the optical centre of a lens and its principal foci.
Used by Edexcel and OCR
Specification 5.5P · Paper 1
The straight reference line passing through the optical centre and the principal foci of a lens.
Revise: Ray Diagrams for LensesIn air, a typical converging lens is thicker in the middle than at its edges. It turns parallel rays towards the principal axis. A typical diverging lens is thinner in the middle and turns parallel rays away from the principal axis.
Specification P5.3c · Paper 2 (Foundation), Paper 4 (Higher)
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The straight line through the optical centre of a lens, perpendicular to the lens.
The principal axis passes through the optical centre of the lens. A ray through the optical centre of a thin lens is shown continuing approximately straight.
Paper 1, Paper 2
Revise: Reflection, Refraction and LensesBoth convex and concave lens diagrams use the principal axis, optical centre, foci and straight rays; their main difference is whether they bring parallel rays together or spread them apart.