Alternative A
Question 2A
You are provided with a rectangular block, a drawing board, drawing sheets, four optical pins and other necessary materials.
Use the diagram above as a guide to perform the experiment.
- Measure and record the width of the block.
- Draw the outline ABCD of the block on a drawing sheet. Mark a point, Q on AB such that AQ = 1.5 cm. Draw the normal RQT as shown in the diagram above.
- Draw a line UQ making an angle i = 100 with the normal RQT.
- Insert pins P1 and P2 on the UQ. Look through the side DC of the block and insert two other pins, P3 and P4 which appear to be in a straight line with the images of P1 and P2.
- Remove the block and mark the positions of P3 and P4. Draw a straight line joining P3 and P4 and extend it to meet DC at E.
- Produce a line UQ to meet DC at point F. Draw a straight line EQ.
- Measure and record the angle, r, length, EF = d, and evaluate tan i, tan r and V = (tan I tan r).
- Using different outlines of the block, repeat the procedure for i = 200, 300, 400 and 500. In each case, measure and record r, d and evaluate tan i, tan r and V.
- Tabulate the results.
- Plot a graph with d on the vertical axis and V on the horizontal axis.
- Determine the slope, s, of the graph.
- State two precautions taken to ensure good results.
[attach the traces to this booklet]
(b) (i) State the condition for the occurrence of total internal reflection at air-glass interface
(ii) State Snell’s law.
Observation
WEAKNESSES: The spacing of the pins is not sufficient.
STRENGTH: Most candidates executed the instructions well.
EXPECTED RESPONSE:
(a) OBSERVATIONS [10]
(i) Width of block W correctly recorded to at least 1 d.p. in cm
(ii) Five complete traces showing at least the normal, incident ray, refracted ray, and emergent ray.
(Deduct ½ mark for each wrong or missing trace)
(iii) Five values of i correctly recorded in degrees.
(Deduct ½ mark for each wrong or missing value)
- Five values of r correctly measured and recorded in degrees and in trend.
Trend: As i increases, r increases.
(Deduct ½ mark for each wrong or missing value)
- Five values of d correctly measured and recorded to at least 1 d.p. in cm
and in trend.
Trend: As i increases, d increases.
(Deduct ½ mark for each wrong or missing value)
- Five values of Tan i correctly evaluated to at least 3 d.p.
(Deduct ½ mark for each wrong or missing value)
(vii) Five values of Tan r correctly evaluated to at least 3 d.p.
(Deduct ½ mark for each wrong or missing value)
(viii)Five values of V = (tan i – tan r) correctly evaluated.
(Deduct ½ mark for each wrong or missing value)
(ix) Composite table showing i, r, d, tan i , tan r and V. NB: (α) If no traces, score zero for i, ii and iii.
(β) If no pin marks, score zero for i, ii and iii
(ɣ) If traces are in one outline, deduct 1 mark for d.i.
(ɳ) If no arrows on at least one trace, deduct ½ mark
GRAPH [06]
- Both Axes correctly distinguished
- Reasonable Scales
- Five points correctly plotted
(Deduct 1 mark for each wrong or missing point)
- Line of best fit
SLOPE [02]
- Large right – angled triangle.
- ∆d correctly determined.
- ∆V correctly determined.
- correctly evaluated.
ACCURACY [01]
Based on slope, s = candidate’s value of w ± 10%
PRECAUTIONS [02]
Award 1 mark each for any 2 correct precautions stated in acceptable tense.
Eg:
- Used well sharpened pencil/ neat traces
- Parallax error avoided in taking readings with protractor/ruler
- Repeated readings shown on table
- Pins separated at about 5 cm apart
- Cleaned surface of the prism.
(b) (i) Condition for the occurrence of total internal reflection at air to glass interface
- Light must travel from glass to air medium
- The angle of incidence(in glass) must be greater than the critical angle (of glass)
(ii) Snell’s Law
The ratio of the Sine of the angle of incidence to the Sine of the angle of refraction is constant for a given pair of media.