A ray strikes the pole of a concave mirror making an angle θ with the principal axis. Using the law of reflection and the fact that the principal axis is the normal at the pole, determine the angle the reflected ray makes with the principal axis. Would your answer change if the mirror were convex? Justify your reasoning.
Generated by claude-sonnet-4-6 · 2026-06-26 01:13 · grounding rag
Model Answer
At the pole, the principal axis acts as the normal to the mirror. By the law of reflection, angle of incidence = angle of reflection. Since the incident ray makes angle θ with the principal axis (the normal), the reflected ray also makes angle θ with the principal axis.
No, the answer would not change for a convex mirror. The principal axis is the normal at the pole for both concave and convex mirrors, so the law of reflection applies identically at the pole regardless of the type of mirror.
Source: Chapter 9, Section 9.1 (Laws of Reflection) and Section 9.2 (Spherical Mirrors — principal axis is normal at the pole)
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Explanation
- The key fact is from the passage: "principal axis is normal to the mirror at its pole." This means the angle with the principal axis = angle of incidence, so reflected angle = θ.
- Examiners expect you to explicitly state why (normal at pole = principal axis) and confirm the answer is the same for both mirror types with a brief reason.
- Do not confuse angle with principal axis with angle with the normal — here they are the same at the pole only.