Question 2
The experiment set above illustrate a practical demonstration carried out in the laboratory. The points QRST are outline of a rectangular glass block. A normal was drawn at a point A, 2 cm from Q to meet ST at B. A straight line DA was drawn such that <DAN = i = 30o. Two pins P1 and P2 were fixed vertically on DA.
Two other pins P3 and P4 were fixed vertically such that they appeared to be in line with images of P1 and P2 when viewed through side TS. A straight line CE was drawn through the positions P3 and P4 to meet TS at C. Another straight line AC was drawn to meet QR at A and TS at C.
- Tabulate your readings.
- Plot a graph with cos θ on the vertical axis and on the horizontal axis.
- Determine the slope, z, of the graph.
- State two precautions that are necessary to ensure accurate results when performing this experiment.
(b) (i) Define principal axis of a concave mirror.
(ii) An object of height 2 cm is 6 cm in front of a concave mirror. Calculate the magnification and the distance of the image produced if the image height is 4.1 cm.
Observation
Part (a) Performance was below average. Most candidates could not measure and evaluate the values of θ to the correct d.p. Many candidates failed in the choice of appropriate scales for graph.
Part (b) Candidates’ responded poorly in this part as many candidates could not define the principal axis of concave mirror correctly
The expected response:
- Five values of AC correctly measured and recorded to at least 1d.p and
- within tolerance of ± 0.1 cm.
- (Deduct ½ mark for each wrong or missing value)
- Five values of BC correctly measured and recorded to at least 1 d.p and within tolerance of ± 0.1 cm.
- (Deduct ½ mark for each wrong or missing value)
- Five values of θ correctly measured and recorded and within
- tolerance of ± 1o.
- (Deduct ½ mark for each wrong or missing value)
- Five values of µ correctly evaluated to at least 2 d.p. (Deduct ½ mark for each wrong or missing value)
- Five values of cos θ correctly evaluated to at least 3 d.p. (Deduct ½ mark for each wrong or missing value)
- Composite table showing at least AC, BC, µ, θ and Cos θ.
- Plot a graph using reasonable scales
- Draw line of best it
- Determine the slope and intercept of the graph
- State any two of the following precautions.
e.g
- Avoided parallax error in protractor/metre rule
- Noted/corrected zero error on metre rule
- Ensured optical pins were vertical/erect/upright
- Ensured reasonable spacing of pins
- Repeated readings stated
b(i) Principal axis is (an imaginary) line joining the centre of curvature (through the principal focus) to the pole.
