In parallax method for the determination of focal length of a concave mirror, the object should always be placed:
at any point beyond the focus of the mirror
between the focus and the centre of curvature of the mirror ONLY
beyond the centre of curvature of the mirror ONLY
between the pole and the focus of the concave mirror ONLY
Correct answer:B
Standard Method
Given: A concave mirror is used in the parallax method to determine focal length.
Find: The correct position of the object for this method.
Principle: In the parallax method, the object is adjusted so that the image formed by the concave mirror coincides with the object itself. The absence of parallax confirms this condition.
For a concave mirror, an object placed between the focus and the centre of curvature forms a real, inverted image beyond , and this arrangement can be adjusted using pins so that no parallax is observed.
If the object is placed elsewhere, the condition of no parallax cannot be satisfied correctly for focal length determination.
Therefore, the object must be placed strictly between the focus and the centre of curvature of the concave mirror.
The correct option is B.
Placing the object between the pole and the focus is incorrect because the image formed there is virtual and cannot be matched by the no-parallax pin arrangement. Instead, place the object between and .
Assuming any position beyond the focus will work is wrong because the parallax method requires a specific real-image condition. The object should be kept specifically between and .
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