Question

In an experiment the image of a distant object formed by a concave mirror is obtained on a screen. To determine the focal length we need to measure the distance between:?

Answer 1

Objective -

To determine the focal length of a concave mirror, by obtaining image of a distant object.

Theory

A concave mirror, like a plane mirror, obeys the laws of reflection of light.
Rays of light from object - The rays of light coming from a distant object e.g. sun or a distant building can be considered to be parallel to each other. When these parallel rays of light fall on a concave mirror along its axis, reflect and meet at a point in front of the mirror, which is called as Principal focus of the mirror.
A real, inverted and very small image size is formed at the focus of the mirror.
Focal Length - The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a 'Real image' of a distant object at its focus, as shown in the figure.

Answer 2

We need to measure the distance between the pole P of the concave mirror and the real image of the object as the rays coming are almost parallel and parallel rays always meet at the focus of the mirror