Physics Paper 1, MAy/June. 2011
 Questions: 1 2 3 4 5 6 Main
Weakness/Remedies
Strength
Alternative a
Question 2

(a)  Diagram

Use the diagram above as a guide to carry out the following experiment.

1. Trace the outline ABCD of the rectangular glass prism on the drawing paper provided.
2. Remove the prism. Select a point N on AB such that AN is about one quarter of AB.
3. Draw the normal LNM.  Also draw a line RN to make an angle θ = 75o with AB at N
4. Fix two pins at P1 and P2 on line RN. Replace the prism on its outline.
5. 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 when viewed through the prism from side DC.
6. Remove the prism and the pins at P3 and P4 Draw a line to join P3 and P4.
7. Produce line P4P3 to meet the line DC at O. Draw a line to join NO.
8. Measure and record the values of MO and NO.
9. Evaluate ф =  and  cos θ.
10. Repeat the procedure for four other values of θ - 65o, 55o, 45o and 35o. In each case, evaluate ф and cos θ
12. Plot a graph with cos θ on the vertical axis and ф on the horizontal axis.
13. Determine the slope, s, of the graph.
14. Sate two precautions taken to ensure accurate results.

(i)   State Snell’s law of refraction.
(ii)  Calculate the critical angle for the glass prism used in the experiment above if
its refractive index is 1.5.

_____________________________________________________________________________________________________
Observation

(This question, based on refraction of light through a rectangular glass prism is not popular among the candidates.  Some candidates that attempted it had problems with their traces.  Some did not attach traces.  Those that attached traces have wrong traces or their traces do not have pin mark.

The graph of Cos θ against ф was poorly attempted as some candidates approximated values of Cos θ to 1 d.p in plotting of graph without reflecting such on the table.  Statement of precautions was poorly rendered

In the short answer question, candidates showed a clear understanding of critical angle.  Candidates were able to calculate critical angle using appropriate formula.  However most of the candidates could not state Snell’s law correctly because they left out a key word: ‘for a given pair of media’ in their definition.

In part (a) the tasks required of the candidates included:

Drawing of five complete traces showing incident, refracted and emergent ray.
Measurement and recording of five values of NO in trend. (Trend: As θ decrease NO increases) Measurement and recording of five values of MO in trend. (Trend: θ decreases as MO increases) Evaluation five values of ф and Cos θ correctly to at least 2 d.p.
Recording θ, NO, MO, ф and Cos θ in composite table.
Plotting at least five points using reasonable scales.
Distinguishing both axes correctly and drawing line of best fit.
Determination of slope using large angle triangle.

State any two of the following precautions in acceptable forms:

• Ensured pins are vertical/erect
• Ensured neat traces/sharp pencil (shown on traces)
• Ensured reasonable spacing of pins (about 4cm apart)
• Avoided parallax error in reading protractor/metre rule.

The expected responses for part b(i) and (ii) are:

(i)   Snell’s law states that the ratio of the Sin  of the angle of incidence to the Sin  of the
angle of refraction is a constant for a given pair of media.

OR

=   a constant for a given pair of media

Where  = angle of incidence
=  angle of refraction

(ii)   n =    C

Sin C  =

C = Sin-1  =  0.6667

C = 41.80o