Science (086) — AI-generated practice question

When rays from a point source placed at the principal focus F₁ of a convex lens pass through the lens, they will emerge as a beam of parallel rays on the other side, parallel to the principal axis. No image is formed (image is at infinity).

Principle involved: According to the rules of ray diagrams, a ray of light passing through the principal focus of a convex lens, after refraction, emerges parallel to the principal axis. As the object is at F₁, all refracted rays become parallel and never converge, so the image is formed at infinity. This is also confirmed by Table 9.4: when object is at focus F₁, "image would not be formed" (i.e., it is at infinity).

Source: Chapter 9, Section 9.3.5 (Image Formation in Lenses Using Ray Diagrams) and Section 9.3.4 (Table 9.4)

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• The key ray rule to quote is: a ray through the principal focus emerges parallel to the principal axis after refraction through a convex lens.
• Since all rays from the point source at F₁ emerge parallel, they never meet → image at infinity → effectively no real image is formed.
• Examiners expect you to: (1) state the observation clearly, (2) name/apply the relevant ray rule, and (3) link it to the table or diagram. All three earn the 3 marks.
• This question tests reversibility of the ray diagram rule: just as parallel rays converge at F₂, a source at F₁ produces parallel rays on the other side.