Science — CBSE Class 10 board question

Ray Diagram:

(Draw a convex lens with its upper half shaded. Show the object AB at 2F₁. Draw two rays from the lower half only — one parallel to the principal axis refracting through F₂, and one through the optical centre — meeting at 2F₂ to form image A'B'.)

Position and Nature of Image:
When the object is placed at 2F₁, the image is formed at 2F₂ — on the other side of the lens, at a distance of 2F. The image is real, inverted, and of the same size as the object.

Observable Difference when Lens is Uncovered:
The image formed is the same in position, nature, and size — but it becomes brighter (more intense) when the full lens is uncovered.

Reason:
Covering the upper half blocks some light rays but the lower half still refracts enough rays to form a complete image. Uncovering the lens allows more light rays to pass through, increasing the brightness (intensity) of the image. The image remains complete in both cases.

Source: Chapter 9, Section 9.3.4 & 9.3.5

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• Examiners specifically award marks for: (1) correct ray diagram with at least two rays from lower half, (2) correct position — at 2F₂, (3) nature — real, inverted, same size, (4) stating brighter image (not a different image), (5) reason linking more light → more brightness.
• Common mistake: students write the image disappears or changes position — it does not. Only brightness changes.
• The ray through the optical centre is the easiest second ray to draw since it passes undeviated even through the lower half.