A concave lens and a concave mirror both diverge a parallel beam of light, yet a concave mirror can form a real image of a distant object while a concave lens cannot. (a) Describe what each does to a parallel beam incident on it, clearly distinguishing between the two. (b) Explain the fundamental physical reason why a concave mirror converges reflected rays to a real focus, whereas a concave lens diverges refracted rays to a virtual focus.
Generated by claude-sonnet-4-6 · 2026-06-26 01:15 · grounding rag
Model Answer
(a) Action on a parallel beam:
- Concave lens: A parallel beam of light, after refraction through a concave lens, diverges (spreads out). The refracted rays appear to come from a point on the same side as the incident light — this is the virtual principal focus (F). The image of a distant object is virtual, erect and diminished.
- Concave mirror: A parallel beam striking a concave mirror is reflected and the reflected rays actually converge to a real point (real principal focus F) in front of the mirror. A distant object therefore forms a real, inverted image at F.
(b) Fundamental physical reason:
A concave lens works by refraction — light bends at two curved surfaces as it passes through the glass. Because the lens is thinner at the centre than at the edges, it bends parallel rays outward, producing only a virtual focus from which rays appear to diverge.
A concave mirror works by reflection — the law of reflection causes each parallel ray to bounce back toward the principal axis. The curved geometry ensures all reflected rays meet at a real point in front of the mirror. No light passes through; it is redirected, allowing actual convergence to a real focus.
Source: Chapter 9, Sections 9.3.3 and 9.3.5
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Explanation
- Examiners look for: (a) correct description of diverging refracted rays → virtual focus for the lens vs converging reflected rays → real focus for the mirror; (b) the key contrast — refraction vs reflection and the geometrical reason (thinner at centre for lens; curved reflecting surface for mirror).
- Use the words real focus and virtual focus explicitly — they carry marks.
- Do not confuse "concave mirror diverges" (it doesn't — that is a convex mirror). The question's opening statement is a common misconception trap; your answer should make clear the mirror converges.
- Keep part (b) tied to the physics: refraction bends light outward in a concave lens; reflection at a concave surface bends light inward.