In parallax method for the determination of focal length of a concave mirror, the object should always be placed
Q. In parallax method for the determination of focal length of a concave mirror, the object should always be placed:

(A) between the focus $(F)$ and the centre of curvature $(C)$ of the mirror ONLY

(B) between the pole $(P)$ and the focus $(F)$ of the concave mirror ONLY

(C) at any point beyond the focus $(F)$ of the mirror

(D) beyond the centre of the curvature $(C)$ of the mirror ONLY

Correct Answer: at any point beyond the focus $(F)$ of the mirror

Explanation

In the parallax method, the focal length of a concave mirror is determined by making the image of an object coincide with a reference point (like a pin or needle) such that there is no relative motion between them when the eye is moved.

For a concave mirror, a real and inverted image is formed only when the object is placed beyond the focus.

If the object is placed between the pole and the focus, the image formed is virtual and erect, which cannot be used for parallax comparison.

Therefore, to obtain a sharp real image suitable for elimination of parallax, the object must be placed at any position beyond the focus of the concave mirror.

Hence, the correct option is:

at any point beyond the focus $(F)$ of the mirror

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