In parallax method for the determination of focal length of a concave mirror, the object should always be placed:
- between the focus (F) and the centre of curvature (C) of the mirror ONLY
- between the pole (P) and the focus (F) of the concave mirror ONLY
- at any point beyond the focus (F) of the mirror
- beyond the centre of the curvature (C) of the mirror ONLY
Answer
(3) at any point beyond the focus (F) of the mirror
Solution
In this experiment the position of the image is located by removing parallax between the image of the object pin and a second (image) pin. The image pin can be placed only at a real image, i.e., in front of the mirror.
A concave mirror forms a real (inverted) image only when the object is beyond the focus:
1v = 1f - 1u; with |u| > |f|, v is negative (real image in front of the mirror).
Object between F and C real image beyond C; object at C image at C; object beyond C real image between F and C.
Object between P and F virtual image behind the mirror, which cannot be located with the image pin.
The object may be placed at any point beyond F (different u give different readings of v, from which f is found).
Answer: (3)