## Friday, 5 September 2014

### OPTICAL FIBRE QUESTIONS AND PROBLEMS

Fibre optics

1. What is meant by total internal reflection?

2. Describe the construction of two sorts of fibres used in fibre optics, illustrating your answer with diagrams.

3. What is the approximate range of diameters of monomode fibres?

4. Why can you bend thin glass fibres without them breaking where if you attempted to do this with thick ones they would shatter?

5. Apart from the danger of breaking why is there a limiting angle for the bending of glass fibres if they are still to be useful?

6. Describe carefully three uses of fibre optics.

7. Explain at least one advantage of using one of the applications that you mentioned in question 1.

8. Does a greater difference between the refractive index of the fibre and that of the surrounding air give a greater or smaller critical angle?

9. In view of your answer to number 3, why is the fibre surrounded by a layer of glass cladding? (at least three reasons are required here).

10. Which has the higher refractive index, the central fibre or the cladding?

11. What is the critical angle between a glass fibre refractive index 1.5 and air, refractive index 1.00?

12. What is the critical angle between a glass fibre refractive index 1.5 and glass cladding? (a) for a difference in the refractive index of the fibre and that of the cladding of 0.05
(b) for a difference in the refractive index of the fibre and that of the cladding of 0.01
(c) for a difference in the refractive index of the fibre and that of the cladding of 0.1

13. A glass fibre of diameter 0.01 mm and 1 km long is used to transmit a light beam. What is the maximum difference in distance between a ray of light that makes no reflections with the fibre walls and one that makes the most possible reflections if the refractive index of the fibre and the cladding are the same as those given in question seven?

14. In the observation of internal organs of the body, for example, why must the bundle of fibres not be twisted relative to each other?

15. Draw an accurate diagram to show a ray of light meeting the centre of the end of a straight glass fibre at an angle of incidence of 40o and entering the glass. For a difference between the refractive index of the core and the cladding prove whether or not this ray will be totally internally reflected and travel down the fibre.

16. Explain with the help of a diagram what is meant by multipath dispersion.