Science (086) — AI-generated practice question

Given: Object distance, $u = -20$ cm; Focal length, $f = -20$ cm (concave mirror)

Mirror formula: $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f}$

$$\frac{1}{v} = \frac{1}{f} - \frac{1}{u} = \frac{1}{-20} - \frac{1}{-20} = -\frac{1}{20} + \frac{1}{20} = 0$$

$$v = \infty$$

Result: The image is formed at infinity.

Physical meaning: When the object is placed at the principal focus (F) of a concave mirror, the reflected rays become parallel and never meet. Therefore, no image is formed at any finite distance — the image is said to be formed at infinity.

Source: Chapter 9, Section 9.2.1 (Table 9.1)

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• Use the sign convention: distances measured in the direction of incident light are positive; opposite are negative. Both $u$ and $f$ are negative for a concave mirror.
• The key result $1/v = 0 \Rightarrow v = \infty$ directly matches Table 9.1: "At F → image at infinity."
• Examiners expect: correct formula substitution, the calculation shown step-by-step, and a clear physical interpretation — all three earn the 3 marks.