A student has to determine the focal length of a concave mirror by obtaining the image of a distant object on a screen. For getting best result he should focus
A student has to determine the focal length of a concave mirror by obtaining the image of a distant object on a screen. For getting best result he should focus
Options
The correct option is B (a well-illuminated distant building).
Marking Scheme
- 1Correct option selected (1 mark)
Hint
For parallel rays to converge at the focus, the object must be at a very large distance (infinity) and must be bright enough to produce a clear image.
Quick Oral Answer
We focus a distant object because the light rays coming from infinity are parallel to each other, and parallel rays converge at the principal focus after reflection from a concave mirror.
Analysis & Explanation
To determine the focal length of a concave mirror, the object must be placed at infinity so that the incident rays are parallel to the principal axis and converge exactly at the principal focus.
- Option B is correct because a distant building is far enough to be considered at infinity, and being 'well-illuminated' ensures that a sharp, bright, and easily visible image is formed on the screen.
- Option A is incorrect because a tree or electric pole may not be sufficiently bright or distinct to form a sharp, high-contrast image.
- Option C is incorrect because the nearest window grills are at a finite, relatively short distance, meaning the image will not form at the focus.
- Option D is incorrect because a candle placed at the edge of the laboratory table is at a finite distance, which does not satisfy the condition of parallel rays from infinity.
Common Mistakes
- 1Choosing window grills (Option C) which are too close to the mirror, leading to an incorrect focal length measurement.
Interesting Facts
The image of a distant object formed at the focus of a concave mirror is always real, inverted, and highly diminished (point-sized).
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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 "Focal Length of Concave Mirror" 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.