(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