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 Physics Paper 2, Nov/Dec. 2007
 Questions: 1 2 3 4 5 6 Main

Weakness/Remedies
Strength

Question 2

(a) Using the above diagram as a guide, carry out the following instructions.

(i) Fix a metre rule on the bench with its graduated side facing up.

(ii) Place the illuminated object at the 0 cm end and the screen at the 100 cm end of the rule such that the distance d between the illuminated object and screen is 100 cm.

(iii) Record the distance d. Also evaluate and record d2 .

(iv) Place and move the converging lens between the illuminated object and the screen until a sharp diminished image of the object is formed on the screen.

(v) Read and record the position l1 of the lens.

(vi) Now move the lens towards the object until another sharp image of the object is formed on the screen.

(vii) Read and record the new position l2 of the lens.

(viii) Evaluate and record L = (l1 – l2), also evaluate L2 and D = d2 - L2.

(ix) Repeat the procedure for four values of d = 85, 75, 65 and 55 cm. Tabulate your readings.

(x) Plot a graph of D on the vertical axis against d on the horizontal axis.

(xi) Determine the slope s, of the graph

(xii) Evaluate k = s
4

(xiii) State two precautions taken to obtain accurate results.
[21 marks]

(b) (i) Explain the statement the focal length of a converging lens is 15 cm.
[2 marks]

(ii) Distinguish between a real image and a virtual image. [2 marks]

_____________________________________________________________________________________________________
Observation

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.

Real Image Virtual Image screen        cannot be received/formed on a screen formed by the intersection of   imaginary rays

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).