A spherical mirror produces a magnification of –3 for an object placed 10 cm from the pole. What type of mirror is it? Where is the image formed, and what does the negative sign of magnification tell you about the image?
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
Type of mirror: It is a concave mirror.
Finding image distance: Using $m = -\dfrac{v}{u}$, with $m = -3$ and $u = -10$ cm:
$$-3 = -\frac{v}{-10} \Rightarrow -3 = \frac{v}{10} \Rightarrow v = -30 \text{ cm}$$
The image is formed 30 cm in front of the mirror (on the same side as the object).
Negative sign of magnification: A negative magnification indicates that the image is real and inverted.
Source: Chapter 9, Section 9.2.4 — Mirror Formula and Magnification
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Explanation
- Why concave? Only a concave mirror can form a real, magnified image (|m| > 1 with negative sign). A convex mirror always gives m between 0 and +1.
- Key formula to remember: $m = -v/u$. Always apply sign convention: object in front → u is negative.
- Examiner expects: identification of mirror type (1 mark), image position with calculation (1 mark), meaning of negative sign (1 mark). Don't skip the calculation step.