Alternative A
Question 2A
You are provided with a drawing board, drawing sheets, four optical pins, a triangular glass prism and other necessary materials.
Use the diagram above as a guide to perform the experiment.
- Pin the drawing paper onto the drawing board.
- Trace the outline ABC of the triangular prism on the drawing paper.
- Remove the prism from its trace. Measure and record the refracting angle A.
- Select a point N on the side AC of the outline. Draw a straight line MO through N and making a glancing angle β=600 with the side AC.
- Fix two pins at P1 and P2 on the line MN. Replace the prism on its outline.
- Look through the side AB of the prism and fix two other pins at P3 and P4 such that they appear to be in a straight line with the images of the pins at P1 and P2.
- Remove the prism and the pins. Join and P4 with a straight line VT and extend it to meet MO at V.
- Measure and record angles αand
. - Evaluate
= (90 - β), Ф = (90 – α) and U = (
-Ф). - Repeat the procedure for four other values of β = 65o, 70o, 75o and 80o. In each case, measure α,γ and evaluate
, Ф and U. - Tabulate the results.
- Plot a graph with γ on the vertical axis and U on the horizontal axis, starting both axes from the origin (0, 0).
- Determine the slope, s, of the graph.
- Determine the intercept, c, on the vertical axis.
- State two precautions taken to ensure accurate results.
Observation
Part (a) was poorly attempted. Many candidates did not attempt it and few that attempted it could not measure angle correctly. Also, some candidates failed to record their data on the table correctly.
The expected response:
(i) Five complete traces showing at least MNO, P3P4W,
angles a, b and γ
(Award ½ mark each)
(ii) Refracting angle, A, correctly measured and recorded in degree
(iii) Five values of a correctly measured and recorded in degree and in trend.
Trend: As b increases, a increases
(Deduct ½ mark for each wrong or missing value)
(iv) Five values of b correctly recorded in degree
(Deduct ½ mark for each wrong or missing value)
(v) Five values of f = [90 - a] correctly evaluated
(Deduct ½ mark for each wrong or missing value)
(vi) Five values of q = [90 - b] correctly evaluated
(Deduct ½ mark for each wrong or missing value)
(vii) Five values of u = [f - q] correctly evaluated
(Deduct ½ mark for each wrong or missing value)
(viii) Five values of g correctly measured and recorded in degree
and in trend
Trend: As b increases, g decreases
(Deduct ½ mark for each wrong or missing value)
(ix) Composite table showing at least b, a, q, f, U and g.
Note: (i) if no trace is attached, award zero for i,ii,iii and iv
(ii) if no pin mark, award zero for i,ii,iii and iv
GRAPH
(i) Both Axes correctly distinguished
(ii) Reasonable Scales
(iii) Five points correctly plotted
(Deduct 1 mark for each wrong or missing point)
(iv) Line of best fit
Note: if graph does not start from origin, (0,0) deduct 1 mark for d.i.
SLOPE (s)
(i) Large right-angled triangle
(ii) Dγ correctly determined
(iii) Du correctly determined
(iv) 
correctly evaluated
INTERCEPT (c)
Intercept, c, on the vertical axis
Correctly shown
Correctly recorded
PRECAUTIONS
e.g.
- Neat traces/sharp pencil
- Avoided parallax error on protractor
- Lens kept upright
- Pin vertical/erect
- Reasonable spacing of pin (at least 4 cm apart)