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# Measurement of the Focal Length of a Convex Lens - Second Method

Mount on one of the stands of the bench a diaphragm with a hole in it across which two fine threads are stretched, or, if more convenient, a piece of fine wire grating, or a pin in a vertical position with its point about the centre of the hole. Place a light behind the hole, taking care that the brightest part of the light is level with the hole and exactly behind it, while the light is as close to the hole as may be.

In the second stand place the lens, fixing it so that its centre is on the same level as that of the hole in the diaphragm, while its axis is parallel to the length of the bench.

In the third stand fix an opaque white screen; a piece of ground glass or unglazed paper is most suitable. For the present purpose the objects can generally be fixed on their respective stands so as to occupy with sufficient accuracy the same relative positions with regard to the zeros of the verniers, and thus the distances between the different objects in question can be obtained at once, by reading the verniers and subtracting.

If the distance between the first and third stand be more than four times the focal length of the lens, the latter can be placed so that there is formed on the screen a distinct image of the object in the first stand. Move the stand carrying the lens till this is the case. Then measure by means of the verniers fixed to the stands, or as described on p. 261, the distance, u, between the object and the first surface of the lens and the distance, v, between the image and the second surface.

Then if we neglect the thickness of the lens the focal length f is given by the formula(1)

The values of v should be observed for at least three different values of u.

Experiment. - Determine by the methods of this and the preceding sections the focal length of the given lens.

Enter results thus:

 (1) Glazebrook, Physical Optics, chap. iv.

Last Update: 2011-03-27