DETERMINATION OF THE FOCAL LENGTH OF A CONVERGING LENS – CASE STUDY AT KADE SENIOR HIGH TECHNICAL SCHOOL USING TWO LENSES LABELLED F = 20CM AND 15CM IN EASTERN REGION OF GHANA – AFRICA.
Abstract
Focal length determination of converging lenses is a major question by WEAC and a major problem for its determination by both students and physics teachers in some SHS in Ghana during this international examination. This research work sorts to investigate two lenses labelled f = 20cm and 15cm by the suppliers and bought by the school as directed by the examining body for the physics practical. The investigation will determine by practical determination of focal length to verify the value quoted by the supplier and given to students to use as the focal length of the lens. The investigation also sort to justify the practical procedure and understanding of students. The methods employed in this work to determine f includes the uv method, far and near distance method and the Bessel’s method. The far and near method gave focal lengths for the 20cm and 15cm converging lens as (19.70 ± 0.24) cm and (14.88 ± 0.09) cm respectively. FL values of (20.12 ± 0.15) cm and (15.04 ± 0.08) cm is the experimental values obtained for the 20cm and 15cm converging lenses using the uv method. The Bessel’s method gave experimental FL values of (19.74 ± 0.32) cm and (14.98 ± 0.04) cm for the 20cm and 15cm after the investigation. This gives a clear indication that, all the converging lenses supplied by suppliers had the focal lengths correctly quoted. One major identified problem with the practical procedure is the use of the far and near method to obtain the focal length (FL) value of the converging lens and then justifying the value obtained through the whole experimental process with the Bessel’s method. An approximate focal length value of 19.5cm can be obtained at a distance of 800cm (8m) and this can’t be said is done by students hence unjustifiable. An exact value of 20cm is obtained at infinity which is far beyond 800cm and therefore questionable as expected of students.
