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Physics Paper 2, Nov/Dec. 2013  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments

Question 13

Question 13

  1. A thin converging lens of focal length, f has an object, O on and perpendicular to its


principal axis.  A real image I is produced by the lens as illustrated in the diagram below.



  1. Given that the principal foci of the lens are the points marked f,use the lens formula to show that f2 = ab
  2. If a = 16 cm and b = 25 cm, determine f.
  3. Calculate the magnification of the image.


  1. Explain the following terms:
  2. principal focus’;
  3. optical centre’;
  4. focal length’.

      Part(a)(i) from the diagram, candidates are to reason out that the object distance u = a+f and the image distance v = b + f.  Then substitute into the lens formula
            1  +  1              1                                                                                                    
            u      v     =     f   to deduce that f2  = ab.


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  

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