To determine focal length of a concave mirror a student obtains the image of a well lit distant object on a screen. To determine the focal length of the given concave mirror he needs to measure the distance between :
To determine focal length of a concave mirror a student obtains the image of a well lit distant object on a screen. To determine the focal length of the given concave mirror he needs to measure the distance between :
Options
To determine the focal length of a concave mirror, a student focuses a distant object onto a screen. Since the object is at a very large distance (effectively infinity), its real and inverted image is formed at the principal focus of the mirror. Therefore, the distance between the mirror (pole) and the screen (where the sharp image is formed) is equal to the focal length of the mirror.
Marking Scheme
- 1Correct Option B selected: 1 Mark
Hint
A distant object acts as an object at infinity. Where does a concave mirror form the image of an object at infinity?
Quick Oral Answer
The image of a distant object is formed at the principal focus of a concave mirror. Therefore, the distance between the pole of the mirror and the screen is its focal length.
Analysis & Explanation
- Option A is incorrect: The distance between the mirror and the distant object is extremely large (infinity) and does not represent the focal length.
- Option B is correct: Since the distant object's image is formed at the focus, the screen must be placed at the focus to capture a sharp image. Thus, the distance between the mirror and the screen is the focal length ().
- Option C is incorrect: The distance between the screen and the object is approximately infinity.
- Option D is incorrect: Measuring the distance to the object is unnecessary and impractical for a distant object.
Common Mistakes
- 1Students sometimes choose Option A, thinking they need to measure the object distance to apply the mirror formula, forgetting that for a distant object, .
Interesting Facts
This method of finding the focal length is called the 'distant object method' and gives an approximate value of the focal length of a concave mirror.
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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.