Measurement of the Focal Length of a Convex Lens - First Method

For this purpose a long bar of wood is employed, carrying at one end a ground-glass screen, fixed at right angles to the length of the bar. A stand, in which the lens can conveniently be fixed with its axis parallel to the length of the bar, slides along it, and the whole apparatus is portable, so that it can be pointed towards the sun or any other distant object.

Place the lens in the stand and withdraw to a dark corner of the laboratory; point the apparatus to a distant well-defined object - a vane seen through a window against the sky is a good object to choose if the sun be not visible - and slide the lens along the bar until a sharply defined image of the object is formed upon the ground glass. Since the object is very distant, the distance of the lens from the screen is practically equal to the focal length, and can be measured either with a tape or by means of graduations on the bar itself. The observation should, of course, be made more than once, and the mean of the measurements taken.