Explain why a concave mirror can be used in a searchlight. When a light source is placed at its principal focus, all the light rays reflecting off the mirror become parallel to the principal axis.

✅ Correct.

The provided solution for parts (c) and (d) is accurate.

• Part (c): The comparison of the two height values (20 m from pinhole camera and 85 m from echo method) is correct, noting the significant difference. The advice to the council to fit a warning light based on the higher value (85 m) exceeding the 20 m minimum is also correct and prudent for safety.
• Part (d): The explanation for using a concave mirror in a searchlight (to produce a parallel beam when the light source is at the principal focus) is correct. The effects of changing the light source's position (parallel, diverging, or converging beam) are also correctly stated.

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