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

We need to determine where the object should be placed in the parallax method for finding the focal length of a concave mirror.

Key Concept: Parallax Method

In the parallax method for a concave mirror, we use the property that a concave mirror forms a real image of a real object. By removing parallax between the object (a pin) and its real image, we can precisely locate the image position and thereby determine the focal length.

Condition for a real image:

A concave mirror forms a real image only when the object is placed beyond the focus (F). Specifically:

- Object at F: Image at infinity (not useful for parallax)

- Object between F and C: Real, inverted, magnified image beyond C

- Object at C: Real, inverted, same-size image at C

- Object beyond C: Real, inverted, diminished image between F and C

If the object is placed between the pole (P) and focus (F), the mirror forms a virtual, erect, magnified image behind the mirror. Virtual images cannot be located by the parallax method using a real pin.

Therefore, for the parallax method to work, the object can be placed at any point beyond the focus - whether between F and C, at C, or beyond C. All these positions produce real images that can be located by removing parallax.

The correct answer is Option 2: at any point beyond the focus (F) of the mirror.