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Physics Paper 1, MAy/June. 2008  
Questions: 1 2 3 4 5 6 Main
General Comments

Question 5

(a) You are provided with a ray box, a converging lens, a screen and other necessary

(i) set up the apparatus as shown in the diagram above;

(ii) measure and record the size xo of the illuminated object;

(iii) place the object at a distance l = 20 cm from the lens;

(iv ) adjust the screen until a sharp image of the illuminated object is obtained
on the screen;
(v) measure and record the size x of the image;

(vi) evaluate d = x and d­¹;

(vii) repeat the procedure for four other values of l = 25, 30, 35 and 40cm;
In each case, determine and record the correspondence values of x, d, and
d­¹. Tabulate your readings;

(viii) plot a graph of d­¹ on the vertical axis against l on the horizontal axis,
starting both axes from the original (0,0);

(ix) determine the slope, s, of the graph. and the intercept, c, on the vertical axis;

(x) determine the values of l for which d­¹ = 0;

(xii) state two precautions taken to obtain accurate results. [21marks]

(b) (i) from your graph, determine the values of d for which l = 24 cm.
(ii) draw a diagram to show how a converging lens may be used to produce
a real diminished image of an object. [2marks]


The candidates were required to:

(i)  set up an experiment as shown in the above;
(ii) measure and record xo and five values x in cm to at least 1 decimal place and in trend.  Trend:  as l increases, x decreases;

(iii) evaluate five values of d and d­¹;     

 (iv) present data on a composite and complete table showing x, l, d and d­¹;    

  (v)  correctly distinguish the graph axes, select reasonable scales, plot the five
        points and draw a line of best fit;

 (vi)  draw a large right angled triangle on the graph line, read d­¹ and ∆l, and
         evaluate slope    =  ∆d2  

(vii) show and read the intercept on the vertical axis;

(vii) deduct value of l for which  d­¹ = O;  

(ix)  state any two of the following precautions in the correct form of  speech:                                          
-     parallax error in reading metre rule avoided;
         -     co-linearity of optical system ensured;
         -     repeated reading (shown on table);
         -     zero error of metre rule noted and corrected;

b (i)  value of  d­¹ corresponding to l = 24;    d­¹ =  l/d
                     -     position of 2f shown object beyond 2f
                     -     ray parallel to principal axis

(ii)   emerging through f and ray through optical centre passing undeviated.
This question was well attempted by candidates.  However, some candidates had difficulties with decimal places and correct placement of units.  Some candidates also measured object and image distances instead of their sizes as demanded by the question.

The b part was fairly attempted by most candidates.


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