TE Enriched Sample (E)

Non-ionizing Medical Imaging 2 76 B Optical fibre Simulation Optical fibre Teaching notes Remind Ss that the refractive index of the cladding is lower than that of the core (to allow total internal reflection). Fig. 2.29 Structure of an optical fibre cladding core refractive index n 1 n 2 Working principle When a light ray enters the core of a fibre, it will eventually strike the core–cladding boundary. If the angle of incidence is greater than the critical angle, the ray will be totally reflected (Fig. 2.30). The ray can then be guided to the other end of the fibre, even if the fibre is curved (Fig. 2.31). Q&A Q: Why the fibre in Fig. 2.31 looks green? Is it due to the green light that the fibre carries? A: No, the fibre looks green because of the reflection on the outer surface. The colour of a good fibre does not depend on the light it carries (which should be totally reflected along the fibre). Fig. 2.30 How an optical fibre works cladding core air total internal ref lection occurs greater than the critical angle Fig. 2.31 A light ray can be guided through a curved optical fibre. cladding core optical fibre 光纖 Try this Basic structure In modern endoscopy, optical fibres are the key elements. Nonetheless, an optical fibre is not complex at all. In fact, it is simply made of a glass fibre core surrounded by a cladding of a slightly lower refractive index (Fig. 2.29). Making your own light guide Prepare some pure gelatin powder and a rectangular container. Follow the steps below. 1. Dissolve a small amount of gelatin in hot water and pour the solution into the rectangular container. 2. Let the solution cool down and a jelly-like block will be formed. 3. Carefully cut a rod of gelatin from the block and your light guide is done. 4. Direct a laser beam into the guide and observe what happens. optically denser (lower n ) (higher n ) Sample © United Prime Educational Publishing (HK) Limited, Pearson Education Asia Limited 2023 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publishers.