| Date | May 2015 | Marks available | 3 | Reference code | 15M.3.SL.TZ2.18 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Explain | Question number | 18 | Adapted from | N/A |
Question
This question is about optic fibre transmission.
Explain, with reference to the critical angle, what is meant by total internal reflection
In an optic fibre the refractive index of the core is 1.62. The refractive index for the cladding is 1.50. Determine the critical angle for the boundary between the core and the cladding.
State one effect of dispersion on a pulse that has travelled along an optic fibre.
Markscheme
for a ray attempting to move from a high to a low refractive index medium;
the phenomena in which the angle of incidence is greater than the critical angle;
(critical angle is) the angle of incidence for which the angle of refraction is 90º / OWTTE;
leading to a reflected but not to a refracted ray;
Award [3 max] for a clearly drawn annotated diagram.
\({\theta _c} = 67.8^\circ \);
Award [2] for a bald correct answer.
pulse width/duration increases / pulse amplitude decreases / colour separation;
Examiners report
Total internal reflection was not usually well explained. Far too many candidates gave disorganised accounts and were unsure which angle was the critical angle.
The value of critical angle was frequently wrong in part (b) as the wrong pair of refractive indices was used.
In (c) there was often confusion between attenuation and dispersion.
