This question was the least popular.  Some candidates mixed up glancing angles with angle of incidence.  Traces were poorly done.  Few did not submit their traces, while some traces submitted did not show the required pin holes or marks.  Majority of the responding candidates were unable to determine dm and em.
Part (i) of question (b) was well attempted while part (ii) was not well attempted.

The expected responses were as follows:

In part (a) the candidates were required to perform the following tasks to score good points:

1. Set up an experiment as shown in the diagram;
2. Draw five complete traces showing at least incident ray, emergent ray and angle of deviation;
3. Record in degree the value of U;
4. Measure and record five values of e;
5. Measure and record five values of d;

(Trend: d decreases, later increases, as Φ decreases)

1. Record data in a composite table showing Φ, e  and d;  
2. Indicate the graph axes,  select reasonable scales, plot the five points;
3. Draw a smooth curve through the points;
4. Show and record minimum deviation dm and the corresponding angle of emergence em;
5. Calculate the refractive index n using the given expression;
6. State any two of the following precautions in acceptable form of speech:
1. Evidence of sharp pencil/neat traces;
2. Evidence of pin spacing;
3. Ensured pins were vertical;
4. Avoided parallax error when reading protractor.

The expected responses to part (b) were as follows:

(i)           Conditions necessary for total internal reflection of light:
              -    Light must be traveling from a denser medium to a less dense medium;    
              -    Angle of incidence in the dense medium must be greater than the critical                                          angle of the dense medium.

    (ii)    Calculation of refractive index, n