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:
S.No. | Distance of flame (cm) | Distance of image (cm)
1 | -90 | +18
2 | -60 | +20
3 | -40 | +24
4 | -30 | +30
5 | -24 | +40
6 | -20 | +60
7 | -18 | +90
8 | -12 | +120
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.
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:
S.No. | Distance of flame (cm) | Distance of image (cm)
1 | -90 | +18
2 | -60 | +20
3 | -40 | +24
4 | -30 | +30
5 | -24 | +40
6 | -20 | +60
7 | -18 | +90
8 | -12 | +120
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.
(a) The focal length of the convex lens used is 18 cm. Justification: From the given observation table, it can be seen that when the object distance is -90 cm, the image distance is +18 cm. This is because the image distance is equal to the focal length of the lens when the object is placed at infinity.
(b) The set of observations that is not correct is set 5. This is because when the object distance is -24 cm, the image distance should be +48 cm (twice the focal length), not +40 cm.
(c) Here is the ray diagram to show image formation for set 1:
Marking Scheme
- 13 marks
Hint
Use the lens equation to analyze the observation table and determine the focal length of the lens.
Quick Oral Answer
The focal length of the convex lens used is 18 cm. This can be determined by analyzing the observation table and identifying the image distance when the object distance is -90 cm.
Analysis & Explanation
The observation table shows the relationship between the object distance and the image distance for a convex lens. By analyzing the table, we can determine the focal length of the lens and identify any incorrect observations. The ray diagram can be used to visualize the image formation process.
Common Mistakes
- 1Calculating the focal length incorrectly
- 2Not identifying the incorrect observation
Interesting Facts
The focal length of a convex lens is the distance between the lens and the image formed when the object is placed at infinity.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2025?
This question carries 3 marks in the CBSE Class 10 Science 2025 examination.
Which chapter does this question come from in Science?
This question is from the chapter "Light — Reflection and Refraction" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Refraction of Light" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2025 paper?
This is a Short Answer question from Section C in the CBSE Class 10 Science 2025 paper.