Alternative B
Question 1
- You are provided with a triangular prism, a drawing board, drawing sheets, four
optical pins, drawing pins and other necessary materials. Use the diagram above as a
guide to carry out the following instructions.
- Draw the outline ABC of the prism provided on a drawing sheet.
- Remove the prism. Measure and record the refracting angle, A, of the prism.
- Draw a line DF to make an angle B = 20 with BA.
- Fix two pins at P1 and P2 on DF. Replace the prism on its outline.
- Look through the side AC 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 f the pins at P1 and P2.
- Remove the prism and the pins.
- Join P3 and P4 with a straight line from G and extend it to the point Q on the side AC of the prism.
- Draw a straight line through P3 and P4 to meet AC at Q.
- Measure the angle y.
- Evaluate θ = (90 – β) and Φ = (90 – γ).
- Repeat the procedure for four other values of B = 30o, 35o, 40o and 50o. In each case, record y and evaluate θ and Φ.
- Tabulate the results.
- Plot a graph with θ on the vertical axis and Φ on the horizontal axis, starting both axes from the origin (0,0). Draw a smooth curve through the points.
- Draw a straight line from the origin to make angle 45o with the horizontal axis and to intersect the curve.
- At the point of intersection, read the values of θ and Φ and record them as θm and Φm respectively.
- Evaluate μ = (sinθ)m/sin(A/2)
- State two precautions taken to ensure accurate results.
-
- Define dispersion of light.
- A ray of light moves from a certain medium into air at an angle of incidence
20o. If the speed of light in the medium is 2.0 x 108
ms-1, calculate the angle of refraction in air.
[speed of light in air =3.0 x 108 ms-1]
Observation
Part (a) Candidates performance was poor. Many candidates did not start the graph from the origin
(0,0), hence they could not deduce θm and Φm
Part (b) Candidates response was unsatisfactory.
The expected response:
3. (a) OBSERVATIONS [07]
(i) The value of refracting angle, A = 60 ± 1o .
(ii) Five complete traces showing at least DF, GQ, β and γ.
(Deduct ½ mark for each wrong or missing trace)
(iii) Five values of β recorded in degrees
(Deduct ½ mark for each wrong or missing value)
(iv) Five values of γ recorded in degrees and in trend
Trend: as β increases γ decreases
(Deduct ½ mark for each wrong or missing value)
(v) Five values of θ = (90 – β) correctly evaluated
(Deduct ½ mark for each wrong or missing value)
(vi) Five values of Φ = (90 – γ) correctly evaluated
(Deduct ½ mark for each wrong or missing value)
(vii) Composite table showing at least β, γ, θ and Φ.
GRAPH [06]
(i) Both axes correctly distinguished (½ mark each)
(ii) Reasonable scales (½ mark each)
(iii) Five points correctly plotted
(Deduct 1 mark for each wrong or missing point)
(iv) Curve of best fit
NOTE: Deduct 1 mark for d.i. if axes do not start from origin (0,0)
STRAIGHT LINE [01]
Straight line making 45° with horizontal axis and intersecting the curve.
DEDUCTION [02]
(i) θm
Correctly shown
Correctly read
(ii) Φm
Correctly shown
Correctly read
EVALUATION [01]
µ = (sinθm)/(sin½A)
Correct substitution
Correct Arithmetic
NOTE: If A = 60 ± 1o is not recorded, award zero for observation (i); but when used to
evaluate
µ, score accordingly.
ACCURACY [01]
Based on the intercept on µ = 1.5 ± 10%
PRECAUTIONS [02]
Award 1 mark each for any 2 correct precautions stated in acceptable tense.
e.g
- Avoided parallax errors on protractor
- Well sharpened pencil used / neat traces
- Optical pins inserted vertically/upright/erect
- Repeated readings shown on table
- Surface of prism cleaned
- Reasonable spacing of pins (about 4 cm apart)
(b)(i) Dispersion of white light
It is the separation of white light into its component/constituent colours.
(ii) If r is the angle of refraction in air,
sin r/sin20o = c/v
sin r = sin 20 x (3.0 x 108)/2 x 108
sin r = 0.513
r = 30.8° ≈ 31°