In parallax method for the determination of focal length of a concave mirror, the object should alwa

Question

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

Professor PC Thomas Classes · Answer

In the parallax method, a real image is required so that its position can be located by removing parallax between the image and a reference pin. A concave mirror forms a real image only when the object is placed anywhere beyond its principal focus ($F$). If it is placed between the pole and the focus, a virtual image is formed behind the mirror, and parallax cannot be removed.