A ray of light after refraction from a convex lens emerges parallel to its principal axis.
(i) Draw a labelled ray diagram to show it.
(ii) In this case, the incident ray before refraction from the lens passes through a point on its principal axis. Name the point.
Generated by claude-sonnet-4-6 · 2026-06-15 06:48 · grounding rag
Model Answer
(i) Ray Diagram:
```
F₁ O F₂
——————————>————————|————————————>——————
Incident ray Lens Emergent ray
(through F₁) (parallel to
principal axis)
```
A ray passing through the principal focus F₁ on the same side as the object, after refraction through the convex lens, emerges parallel to the principal axis (as per Fig. 9.14(a)).
(Students must draw: convex lens, principal axis, incident ray directed through F₁, refracted ray parallel to axis, with labels: Convex lens, F₁, F₂, Principal axis.)
(ii) The incident ray passes through the principal focus (F₁) of the convex lens.
Source: Chapter 9, Section 9.3.5
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Explanation
- Part (i) tests whether you know ray rule (ii) for lenses: a ray through the focus on the incident side emerges parallel to the principal axis. Always label F₁, F₂, O (optical centre), and the lens.
- Part (ii) directly names that point as the principal focus (F₁). Many students write "focus F₂" (wrong side) — remember F₁ is on the same side as the object.
- Since this is a 2-mark question (1 mark each), keep the diagram neat and the written answer to one line.