Out of the two lenses, one concave and the other convex, state which one will diverge a parallel beam of light falling on it. Draw a ray diagram to show the principal focus of the lens.
Generated by claude-sonnet-4-6 · 2026-06-15 06:48 · grounding rag
Model Answer
Concave lens will diverge a parallel beam of light falling on it. It is also called a diverging lens.
Ray Diagram — Principal Focus of a Concave Lens:
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----\---|---/------ Principal axis
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F₁ O
(Virtual focus, same side as incident rays)
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Parallel rays, after refraction through the concave lens, appear to diverge from the principal focus F₁, located on the same side as the incident light.
Source: Chapter 9, Section 9.3.3 — Refraction by Spherical Lenses
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Explanation
- The question has two parts: (i) identifying which lens diverges — concave (1 mark), and (ii) drawing the ray diagram showing principal focus (1 mark).
- Key examiner expectation: the diagram must show parallel rays appearing to diverge from F₁ on the same side as the incident light — this is what distinguishes a concave (virtual) focus from a convex (real) focus.
- Always label the optical centre O, principal focus F, and the principal axis in the diagram.
- Don't confuse: convex = converging (rays meet at F on the other side); concave = diverging (rays appear to come from F on the same side).