A student has obtained a point image of a distant object using the given convex lens. To find the focal length of the lens he should measure the distance between the:
A student has obtained a point image of a distant object using the given convex lens. To find the focal length of the lens he should measure the distance between the:
Options
To find the focal length of a convex lens using a distant object, the student should measure the distance between the lens and the screen only.
Marking Scheme
- 1Correct option B selected (1 mark)
Hint
Think about where the parallel rays from a very distant object converge after passing through a convex lens.
Quick Oral Answer
The image of a distant object formed by a convex lens is real, inverted, and highly diminished (point-sized) at the principal focus.
Analysis & Explanation
When light rays from a distant object (at infinity) fall on a convex lens, they are parallel to the principal axis. After passing through the lens, these rays converge at the principal focus. Therefore, a sharp, point-sized image of the distant object is formed on a screen placed at the focus. The distance between the optical centre of the lens and the screen (focus) is equal to the focal length of the lens. Hence, option B is correct.
- Option A is incorrect because the object is at infinity, so measuring the distance to the object is not required or feasible.
- Option C and D are incorrect because they involve the object distance, which is not needed to find the focal length in this setup.
Common Mistakes
- 1Confusing the distance between the object and the lens with the focal length.
- 2Measuring from the object to the image instead of the lens to the screen.
Interesting Facts
This method is called the 'distant object method' and provides an approximate focal length of a convex lens quickly without complex setups.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2015?
This question carries 1 mark in the CBSE Class 10 Science 2015 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 2015 paper?
This is a MCQ question from Section B in the CBSE Class 10 Science 2015 paper.