A concave mirror is placed facing the Sun, and a piece of paper is held at its focus. After some time, the paper catches fire. Using the properties of reflection of parallel rays by a concave mirror and the concept of the principal focus, explain why this happens.
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
The Sun is very far away, so rays of sunlight reaching the concave mirror are essentially parallel to its principal axis. According to the properties of reflection by a concave mirror, all rays parallel to the principal axis, after reflection, converge at the principal focus (F).
When the paper is held at the focus, all the reflected rays meet at that single point. This concentrates a large amount of solar energy onto a very small area of the paper, raising its temperature to its ignition point. Hence, the paper catches fire.
Source: Chapter 9, Section 9.2.2 – Representation of Images by Spherical Mirrors; Table 9.1
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Explanation
- Key ray property to state: Rays parallel to the principal axis converge at focus F after reflection — this is the core point (1 mark).
- Link to focus: Paper held at F receives all reflected rays at one point (1 mark).
- Consequence: Concentrated energy → high temperature → paper ignites (1 mark).
- Don't forget to mention that sunlight rays are parallel because the Sun is at infinity — this justifies using the parallel-ray property.
- Table 9.1 confirms: object at infinity → image at focus F, real, point-sized — exactly the situation here.