Very few candidates attempted this question and almost all the responding candidates  missed it.  Performance was poor as most candidates did not know that u = a + f and
v = b+ f
 
(ii)   This part was fairly well attempted.  Most candidates were able to calculate f.
(iii)   This was fairly well attempted.

     Part (b):  Few candidates could not properly explain (i),(ii) and (iii).  However,
     performance was average.

     The expected answers are:

      (a)(i)                u  =  a + f                                                                                         
                               v  =  b + f                                                                                      
                                   =     +                                                                                      
                                 =   +                                                                              
                                 =   
                                        (  )
                                     (  ) =  )                                                      
                                        ab +af +bf + f2 =  bf + f2 + af + f2

                                                           f2     =  ab                                                                                                       

(ii)                       2      =  ab
                                        =   16  x  25                                                                                    
                                        
                                    f    =  20cm                                                                                           

(iii)                       Magnification   =                                                                               

                                                   =                                                                         
                                                   =  1.25                                                                                  
                                                             
b(i)      Principal focus of a lens/ mirror is the point on the principal axis to which all rays parallel and close to the    principal axis either  converge or (appear) to diverge after refraction/ reflection by the lens/ mirror 
 
  (ii)     Optical centre is a point through which rays of light pass without deviation by the lens   
  (iii)     Focal length is the distance between the optical centre/ centre of curvature and the principal focus of a lens/ mirror .Additional information: e.g. f =   ; diagrams e.t.c