Observation table recording distance of flame from the lens and distance of the image from the lens.
A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular form as given below :
Analyse the observation table and on the basis of your analysis only, answer the following questions (without doing any calculations) :
(a) What is the focal length of the convex lens used ? Give reason to justify your answer.
(b) Which one of the sets of observations is not correct and why ?
(c) Draw ray diagram to show image formation for any correct set of observation.
Generated by claude-sonnet-4-6 · 2026-06-15 06:51 · grounding stimulus
Model Answer
(a) Focal length = 15 cm
Reason: In rows 4 and 5, when object distance = 30 cm, image distance = 30 cm (equal distances, so object is at 2F). This means 2F = 30 cm, hence F = 15 cm.
(b) S.No. 8 (u = −12 cm) is incorrect. Since the object is placed at 12 cm, which is less than the focal length (15 cm), a convex lens cannot form a real image on the screen; the image would be virtual.
(c) Ray diagram for S.No. 4 (object at 2F, u = −30 cm):
```
F 2F 2F F
-----|----|----|lens|----|-----
[obj] [img]
```
Two rays drawn: one parallel to principal axis refracting through F, and one through optical centre passing undeviated — image forms at 2F on the other side, real, inverted, same size.
Source: Science Lab Manual / Light – Reflection and Refraction, Convex Lens experiment
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Explanation
- For (a): The key observation is symmetry — when u = v, the object is at 2F. Rows 4 & 5 confirm this without any calculation.
- For (b): Any object within the focal length of a convex lens gives a virtual image (cannot be caught on screen), so u = −12 cm < f = 15 cm is impossible in this experiment.
- For (c): CBSE expects a neat, labelled ray diagram with at least two rays; examiners award 1 mark for the diagram, so keep it clear and label F, 2F, object, image.