Physics Paper 3, May/June 2014

Questions:	1	2	3	Main

General Comments
Weakness/Remedies
Strength

Question 2
(a) You are provided with a ray-box with a cross-wire, a converging lens and its holder, a mounted plane mirror, a metre rule and a screen. Clamp the metre rule to the table. Erect the lens in its holder at position A which is near the 50 cm mark of the metre rule. Mark and maintain the position A of the lens throughout the experiment. Place the plane mirror behind the lens. Place the ray-box on the opposite side of the lens as illustrated in the Fig. 1 above and switch it on. Adjust the position of the ray-box until a sharp image of the cross-wire is formed on the box. Mark the position of the ray-box as P. Remove the plane mirror and set it aside. Remove the ray-box and place it on the opposite side of the lens at a distance of x = 25 cm from the lens as illustrated in Fig. 2. Measure and record the height do of the cross-wire. Place the screen beyond P and adjust its position until a sharp image of the cross-wire is formed on it.\ Measure and record the distance y between P and the screen. Measure and record the height, di of the image. Repeat the procedure for four other values of x = 30 cm, 35 cm, 40 cm and 45 cm respectively. In each case, read and record the corresponding values of y and di. Tabulate your readings. Plot a graph with di on the vertical axis and y on the horizontal axis. Determine the slope, s, of the graph. Given that di = ( ) y deduce the value of K. State two precautions taken to obtain accurate results. (b) (i) Define principal focus of a converging lens. (ii) Draw a ray diagram to show how a converging lens forms a real image of unit magnification.
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Observation
Part (a) This question was fairly popular among the candidates and it was poorly attempted. Most responding candidates could plot the graph. The determination of K was a problem to most candidates. Part (b): Was poorly attempted by most candidates. They could neither define properly the principal focus. Candidates are expected to: (i) Measure and record do correctly to at least 1 d.p in cm. (ii) Correctly measure and record five values of x to at least 1 d.p in cm. (iii) Correctly measure and record five values of y to at least 1 d.p in cm and in trend (Trend: As x increases, y decreases) (iv) Correctly measure and record five values of di to at least 1 d.p. in cm and in tre (v) Show composite table containing x, y, and di. Plot a graph using reasonable scales: - draw line of best fit. - determine the slope of the graph. - deduce the value of K given that di = ( ) y - state any two of the following precautions. e.g - collinearity of object, lens and screen ensured. - Parallax error avoided on metre rule - Zero error avoided/corrected on metre rule - Repeated readings (shown on table) In part (b), the expected answers are: (i) The principal focus of a converging lens is the point on its principal axis where all rays parallel and close to it converge after refraction through the lens. To score (i) Object must be at 2f. (ii) Correct image must be at 2f on the other side. (iii) Any two correct rays from the object intersecting to form the image.