K.C.S.E Physics Q & A - MODEL 2011PP2QN16

K.C.S.E Physics Q & A - MODEL 2011PP2QN16

​(a) State the meaning of the term “principal focus” as applied in lenses. 
(b) You are provided with the following apparatus to determine the focal length of a lens:
- a biconvex lens and lens holder.
- alit candle.
- a white screen.
- a metre rule

​(i) Draw a diagram to show how you would arrange the above apparatus to determine the focal length of the lens 
(ii) Describe the procedure you would follow. 
(iii) State two measurements that you would take. 
(iv) Explain how the measurements in (iii) would be used to determine the focal length. 
(c) An object is placed 30cm in front of a concave lens of focal length 20cm.
Determine the magnification of the image produced.

answers

​(a) The point at which rays close to and parallel to the principal axis converge or seem to diverge from after striking the lens;

​(ii) Candle is placed at a certain distance from the lens. The distance between the screen and the lens is adjusted until a sharp image is focused on screen.
     (iii) The distance of candle from lens (U) is measured;
     The distance of screen from lens (V) is also measured;