Optics Sign Convention Traps
Test yourself on the Cartesian sign convention for mirrors and lenses — the rules that quietly decide whether a JEE Main optics answer comes out right or upside down.
Fifteen questions cover where distances are measured from, why u is always negative for a real object, which elements get a positive focal length, the two formulas that differ by a single sign, and what a negative magnification actually tells you.
Every question is built around a mistake students actually make, not a definition to recite.
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Sample questions3 of 15 shown
Q1
A student writes the object distance for a spherical mirror by measuring it from the centre of curvature, because "C is the natural reference point". Where must all distances actually be measured from in the Cartesian sign convention?
Q2
While solving a lens problem, a student measures distances starting from the focal point of the lens. Which choice of origin is demanded by the sign convention for a lens?
Q3
A distance is measured in the direction opposite to that of the incident light. A student calls it positive "because length can't be negative". What sign should it carry?
What This Quiz Covers
- Where u, v and f are measured from
- Why a real object always gives negative u
- Focal length signs: concave against convex
- 1/v + 1/u = 1/f against 1/v − 1/u = 1/f
- m = −v/u for mirrors, m = v/u for lenses
- What the sign of the magnification means
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