A candle is moved from a point far beyond the centre of curvature of a concave mirror steadily toward its focus. How does the size of the image change during this movement? What is the nature of the image when the candle is placed exactly at the focus?
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
Change in size of image:
As the candle moves from far beyond C toward F, the image size continuously increases. Specifically:
- Beyond C → image is diminished
- At C → image is the same size as object
- Between C and F → image is enlarged (larger than object)
Throughout this movement, the image remains real and inverted.
At the focus:
When the candle is placed exactly at the focus F, the reflected rays become parallel and do not meet. The image is formed at infinity and is highly enlarged (practically, no image is formed on a screen).
Source: Chapter 9, Section 9.2.1, Table 9.1
---
Explanation
- Examiners expect students to trace the image size through each position (beyond C → at C → between C and F → at F) using Table 9.1.
- The key phrase for "at F" is "at infinity" — state this clearly; some students incorrectly say "no image forms" without explaining why (rays emerge parallel).
- Mentioning that the image remains real and inverted throughout the movement earns an extra mark.
- Keep the answer structured: one part for the moving candle, one part for the candle at F.