The candidates were expected to:

(i) set up an experiment as shown in the diagram above

(ii) measure and record five values of l1 – l2 to at least 1 decimal place and in trend. (Trend: as l2 less than l1)

(iii) evaluate five values of L and D L2;

(iv) record data in a composite table showing at least d, l1 l2, L, L and D;

(v) indicate the graph axes, select reasonable scales, plot the five points correctly and draw a line that best fits the points;

(vi) draw a large right angled triangle on the graph line;

(vii) read and record ΔD and Δd also evaluate ΔD;
                                                                                 Δd
(viii) correctly evaluate k = s
                                                 4
(ix) state any two of the following precautions in acceptable form of speech.
- Parallax error on reading metre rule was avoided
- The optical component correctly aligned;
- Vertical placement of lens ensured;
- Repeated readings (must be shown on table).

b (i) This means that the distance between the optical centre (or middle of lens) of
the lens and its principal focus is 15 cm.

Many candidates attempted this question.  Many of those who did could not obtain very accurate readings and hence did poor graph work.

Part b (i) was poorly attempted by most candidates unlike b(ii).