A student has to determine the focal length of a convex lens by obtaining the image of a distant object on a screen. Which of the following measures should he take to obtain better results? a. Select a lens of small diameter (say 3 cm). b. Select a lens of larger diameter (say 5 cm). c. Select an object very far from the lens. d. Select an object far, but not very far, from the lens. e. Keep all the lights of the room on. f. Keep minimum lights of the room on.
A student has to determine the focal length of a convex lens by obtaining the image of a distant object on a screen. Which of the following measures should he take to obtain better results? a. Select a lens of small diameter (say 3 cm). b. Select a lens of larger diameter (say 5 cm). c. Select an object very far from the lens. d. Select an object far, but not very far, from the lens. e. Keep all the lights of the room on. f. Keep minimum lights of the room on.
Options
The correct answer is (B): b, d and f.
Reasoning for each option:
• (b) Select a lens of larger diameter (say 5 cm) ✓ — A larger aperture gathers more light, producing a brighter, sharper image on the screen.
• (d) Select an object far, but not very far, from the lens ✓ — Practically, an object at a large but finite distance still sends nearly parallel rays, giving a clear image close to the focus.
• (f) Keep minimum lights of the room on ✓ — A darker room reduces stray/ambient light, making the image on the screen more distinct and easier to measure.
Why the others are wrong:
• (a) Small lens diameter — less light, dim and blurred image.
• (c) Object very far — although ideal for parallel rays, it is impractical to place a screen at exactly the focal point in a classroom.
• (e) All lights on — ambient light washes out the image on the screen.
Hence, the best combination is b, d and f.
Marking Scheme
- 11 mark for selecting the correct option (B).
Hint
For a bright and sharp image, you need: large aperture lens, an object that is 'effectively' at infinity, and a dark room.
Quick Oral Answer
A convex lens of larger diameter gathers more light, so the image of a distant object on the screen is brighter. Keeping the room dark reduces stray light, making the image more visible. A far-but-finite distance acts as a practical 'infinity' for the experiment.
Analysis & Explanation
To find the focal length using a distant object, the image must be sharp and bright. A larger diameter lens (option b) captures more rays, making the image brighter. A finite but large object distance (option d) is a practical compromise — the object is 'almost' at infinity, so its image forms almost at the focal point. Dimming the room (option f) improves contrast on the screen, allowing the image position to be measured accurately. These are standard practical considerations highlighted in the NCERT activity for determining focal length.
Common Mistakes
- 1Choosing option (D) thinking 'all lights on' is helpful — actually it reduces image contrast.
- 2Choosing option (A) which combines small lens + very far object + all lights — all three are unfavourable.
Interesting Facts
Astronomers use the same principle — the focal length of a telescope's objective lens is found by focusing the image of a very distant star.
A larger lens diameter not only makes the image brighter but also improves the resolving power of the lens.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2012?
This question carries 1 mark in the CBSE Class 10 Science 2012 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 "Determining focal length of a convex lens using a distant object" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2012 paper?
This is a MCQ question from Section B in the CBSE Class 10 Science 2012 paper.