Science (086) — AI-generated practice question

Mirrors and lenses work on different optical principles — mirrors work by reflection, while lenses work by refraction. Due to this fundamental difference, the sign conventions and the geometry of image formation differ.

In a mirror, the object and image are both on the same side (in front of the mirror), so distances are measured from the pole, and the formula is $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f}$.

In a lens, the object and image are on opposite sides, with all distances measured from the optical centre. This geometric difference changes the algebraic relationship, giving $\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$.

Thus, despite both involving curved surfaces, the different physical processes and measurement conventions make a single formula unsuitable for both.

Source: Chapter 9, Sections 9.2.4 and 9.3.7

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• Examiners expect you to identify reflection vs. refraction as the root cause.
• Then explain the geometric difference — same side vs. opposite sides — which changes the sign relationship between u, v, and f.
• Avoid just quoting the two formulas without explaining why they differ; that won't fetch full marks.
• Key terms to use: pole, optical centre, sign convention, reflection, refraction.