A student obtained a sharp image of the grills of a window on a screen using a concave mirror. His teacher remarked that for getting better results a well lit distant object (preferably the sun) should be focussed on the screen. What should be done for this purpose?
A student obtained a sharp image of the grills of a window on a screen using a concave mirror. His teacher remarked that for getting better results a well lit distant object (preferably the sun) should be focussed on the screen. What should be done for this purpose?
Options
To obtain a sharp image of a well-lit distant object (like the Sun) instead of nearby window grills, the student needs to move the mirror slightly towards the screen (or move the screen closer to the mirror).
Initially, the window grills are at a finite distance (beyond ). The image of an object placed beyond is formed between and .
When focusing a much more distant object like the Sun (which is at infinity), the image is formed exactly at the principal focus . Since the focus is closer to the mirror than the region between and , the distance between the mirror and the screen must be decreased. Therefore, the mirror should be moved slightly towards the screen.
Marking Scheme
- 1Correct Option B selected: 1 Mark
Hint
An object at infinity forms an image closer to the mirror (at ) compared to an object at a finite distance (between and ).
Quick Oral Answer
As the object distance increases towards infinity, the image distance decreases and approaches the focal length. Thus, the screen must be brought closer to the mirror.
Analysis & Explanation
- Option A is incorrect: Moving the screen away from the mirror increases the distance, whereas the focus lies closer to the mirror than the initial image position.
- Option B is correct: Moving the mirror towards the screen decreases the distance between them, aligning the screen with the principal focus where the image of the Sun is formed.
- Option C & D are incorrect: Moving both together does not change the relative distance between the mirror and the screen in a controlled manner to find the focus.
Common Mistakes
- 1Students often think that a more distant object requires moving the screen further away, whereas the image actually moves closer to the mirror (towards the focus) as the object moves further away.
Interesting Facts
As an object moves from infinity towards the pole of a concave mirror, its real image moves away from the focus towards infinity.
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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 "Spherical Mirrors" 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.