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Science (086) — CBSE Class 10 board question

Q1. [1] § 9.2.3 Sign Convention for Reflection by Spherical Mirrors § 9.3.6 Sign Convention for Spherical Lenses
An object is placed in front of a convex mirror at infinity. According to the New Cartesian Sign Convention, the sign of the focal length and the sign of the image distance in this case are respectively :
  1. (a) +, −
  2. (b) −, +
  3. (c) −, −
  4. (d) +, +
Previously asked in CBSE board exam
2023 31/2/1 Q13
Generated by claude-sonnet-4-6 · 2026-06-15 06:49 · grounding rag
Model Answer

(d) +, +

For a convex mirror, the principal focus lies behind the mirror (to the right of pole), so focal length $f$ = positive. When object is at infinity, image forms at focus F, also behind the mirror, so image distance $v$ = positive.

Explanation

Key points: Under New Cartesian Sign Convention, distances measured to the right of the pole are positive. A convex mirror's focus is behind the mirror (right side) → $f$ = +. Object at infinity gives image at F (behind mirror) → $v$ = +. This is directly supported by Example 9.1 where $f = +1.50$ m and $v = +1.15$ m for a convex mirror, and Table 9.2 which states the image forms "at focus F, behind the mirror" when object is at infinity.

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