The candidates were required to:
(i)complete five traces of the ray paths;
(ii)measure five values of NO correctly and record to at least 1 decimal place in cm and in trend.
Trend: NO decreases as Φ increases;
(iii)measure five values of MO and record to at least 1 decimal place in cm and in trend.
Trend: MO decreases as Φ increases;
(iv)evaluate five values of Φ correctly and record to at least 2 decimal place in cm;
(v)read and record five values of cos 0 correctly to at least' 3 decimal place;
(vi)record data on a composite table showing NO, MO, Φ and cos Φ.
(vii)correctly distinguish the graph axes, choose reasonable scales, plot five points and draw a line that best fits into the points;
(viii)draw a large right angled triangle on the graph line, read Δ cos θ and
Δcos Φ/Δθ
(ix)obtain a value for slope = the refractive index of glass;
(x)state any two of the following precautions in the correct form of speech;
-I ensured pins are vertical;
-I ensured neat traces/sharp pencil (as shown on traces);
-I ensured reasonable spacing of pins (as shown on traces, spaced about 4 cm apart);
-I avoided parallax error in reading the protractor or metre rule.
(b)(i)Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a give pair of media.
OR
sin i = a constant for a given pair of media.
sin r
where i = angle of incidence
r = angle of refraction
(ii) 1/ sin c = reflection index
1/ sinc = 1.5
sin c = 1/ 1.5 = 0.6667
c = sin-1 0.667
c = 41.80
This ray tracing experiment was also very popular among the candidates. The performance was however poor.
Most candidates could not present good, neat and complete traces. Most candidates were able to measure the value of Φ and θ correctly.
However, some candidates could not correctly read cos Φ from tables. Also, the graph posed a lot of challenges to most candidates due to the value of cos Φ involved. The candidates, however, were able to determine the slope of their graph.
The precautions were well stated
In part (b) most candidates were able to state Snell’s law. A few candidates could only state sin i = n without
sin r
defining i as the angle of incidence, r as angle' of
refraction and n as refractive index or a constant.
The numerical in part b (ii) was poorly handled. Some candidates could only manage to recall 1/ sin c = n but could not proceed further to determine c.
