A bulb is placed behind a hole with a cross wire on a cardboard so as shown in figure below. A lens on a lens holder is placed between a mirror and the cardboard.
The cardboard together with the source of light is moved along the metre rule until a sharp image of the cross wire is formed along the cross wire object as shown. The figure shows two rays emerging from the point source towards the mirror through the lens
The lengths f gives the focal length of the lens.
Explanation
The ray striking the mirror are reflected back along the same paths of the incidence so that the image of the source coincides with the source itself. This image can be received on a screen placed at the same position as the source as shown.
If both the lens and the mirror are perfectly vertical or parallel to each other, the image perfect coincides with the illuminated object hole so that it cannot be seen, it is therefore necessary to tilt either the lens or the mirror a little so that the image can be mapped besides the hole.
some equivalent arrangement is as shown.
In the above arrangement, the object pin is moved towards the lens or away from it until when it coincides with it’s inverted image and this occurs when the pinhead is vertically above the center of the lens.
At a point where the object and the image perfectly coincides, there is no relative motion between them as the eye is moved perpendicular to them and instead, they move together as one.
The distance between the pin and the lens is then measured as the focal length of the lense.
NB: Focal length increases as thickness of the lens decreases. This is because thick lenses refracts and deviates light more sharply than a thin lenses. Therefore, rays emerging from thick lens tends to converge earlier because because of the sharp bending in the lens.
