Suppose you have focused on a screen the image of candle flame placed at the farthest end of the laboratory table using a convex lens. If your teacher suggests you to focus the parallel rays of the sun, reaching your laboratory table, on the same screen, what you are expected to do is to move the:
Suppose you have focused on a screen the image of candle flame placed at the farthest end of the laboratory table using a convex lens. If your teacher suggests you to focus the parallel rays of the sun, reaching your laboratory table, on the same screen, what you are expected to do is to move the:
Options
The correct option is (a).
Initially, the candle flame is at a finite distance (at the farthest end of the table), so its image is formed at a distance greater than the focal length (). When focusing parallel rays from the sun (which is at infinity), the image is formed exactly at the principal focus (). Since the focus is closer to the lens than the previous image position, the distance between the lens and the screen must be decreased. Therefore, the lens must be moved slightly towards the screen.
Marking Scheme
- 11 mark
Hint
As the object moves further away (from a finite distance to infinity), the image moves closer to the lens (towards the focus). Think about how you should adjust the lens-to-screen distance to match this.
Quick Oral Answer
To focus a more distant object, the screen must be brought closer to the lens, or the lens must be moved closer to the screen, because the image distance decreases as the object distance increases.
Analysis & Explanation
- Why (a) is correct: For a convex lens, as the object moves further away towards infinity, the image moves closer to the principal focus (). The candle flame is at a finite distance, so its image is formed beyond . The sun is at infinity, so its image is formed exactly at . To capture this new image on the same screen, the distance between the lens and the screen must be reduced, which is achieved by moving the lens slightly towards the screen.
- Why (b) is incorrect: Moving the lens away from the screen increases the distance, which would defocus the image of the sun.
- Why (c) and (d) are incorrect: Moving the lens towards the sun or moving both together does not systematically adjust the lens-to-screen distance to match the focal length.
Common Mistakes
- 1Thinking that the image of the sun will form at the same position as the candle flame.
- 2Moving the lens away from the screen instead of towards it.
Interesting Facts
A convex lens can concentrate sunlight so intensely at its focus that it can burn paper, just like a concave mirror does!
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2016?
This question carries 1 mark in the CBSE Class 10 Science 2016 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 by Spherical Lenses" from CBSE Class 10 Science.
Has this question appeared in other CBSE Science papers?
Yes, a similar question appeared in the 2016 CBSE Class 10 Science paper.
What type of question is this in the CBSE Class 10 Science 2016 paper?
This is a MCQ question from Section B in the CBSE Class 10 Science 2016 paper.