How do you find the focal length of a concave mirror? (AS1)

A mirror is called a concave mirror if its reflecting side bulges inwards. A concave mirror often referred as converging mirror as it converges the light rays after reflection.

Focus is the point where the incident parallel rays converge, after reflection from the mirror, at a distance of R/2 from the mirror where R = radius of curvature.
Focal length is defined as the image distance when the object is at infinity.

Aim: To find the focal length of a concave mirror

Apparatus required: concave mirror, meter scale, a white screen, Mirror holder, a distant object

Procedure: Set up the apparatus near a window so to get a distant object. Mount the concave mirror on the mirror holder. Now place the meter scale on the table and keep the mirror at a distance of 5 cm mark on the meter scale facing the object. Place the white screen in between the mirror and the distant object through the meter scale. Now move the screen until you get a sharp, diminished & inverted image of the distant object. Make sure the mirror & the screen have the same height. Mark the point on the scale where you get the desired image. The difference between the mirror mark & the screen mark will give the focal length.

Observation: