Which of the following can be used to find the focal length of a convex lens ?
Which of the following can be used to find the focal length of a convex lens ?
Options
Official answer
Light from the sun can be used to find the focal length of a convex lens because the sun is at an infinite distance, providing parallel rays that converge at the principal focus.
Marking scheme
- 1.Correct option selection (1 mark)
Hint
Look for the object that is furthest away.
Quick oral answer
We use the Sun because it is at infinity, and rays from infinity converge at the principal focus after passing through a convex lens.
Common mistakes
- 1.Choosing a nearby object like a window which will not produce a sharp image exactly at the focus.
Analysis and explanation
To accurately find the focal length, we need incident rays that are parallel to the principal axis. Objects at a very large distance (infinity) produce such rays. The Sun is the most reliable distant source. As per the logic in Activity 9.2 (applied to lenses), these rays converge at the focus. A tree next to a window or a laboratory window might not be far enough to be considered 'at infinity' for a lens with a standard focal length.
Interesting facts
Using the Sun to find focal length can actually burn the screen (paper) due to the concentration of heat energy, as mentioned in Activity 9.2.
Question details
Q27 of 41 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2011
- Section
- Section B
- Marks
- 1
- Type
- MCQ
- Chapter
- Light — Reflection and Refraction
- Topic
- Refraction by Spherical Lenses
- Bloom's level
- Apply
- Syllabus topic
- Refraction by spherical lenses; Focal length
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Frequently asked questions
How many marks does this question carry in CBSE Class 10 Science 2011?
This question carries 1 mark in the CBSE Class 10 Science 2011 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.
What type of question is this in the CBSE Class 10 Science 2011 paper?
This is a MCQ question from Section B in the CBSE Class 10 Science 2011 paper.