TE Enriched Sample (E)

75 Another common imaging method that uses non-ionizing radiation is endoscopy . Its working principle is based on the total internal reflection of light. Let’s do a quick revision on this principle first. A Total internal reflection Snell’s law and critical angle When a light ray travels in a medium and is incident on a boundary with the next medium, it is partly refracted and partly reflected. The refraction of light obeys Snell’s law: n 1 sin θ 1 = n 2 sin θ 2 If n 1 > n 2 , and θ 1 is larger than the critical angle, all the light will be reflected. The critical angle c can be found by sin sin sin n c n c n n 90° 1 2 1 2 & = = Total internal reflection Total internal reflection occurs on a boundary when 1. the light ray is directed towards an optically less dense medium, and 2. the angle of incidence is greater than the critical angle. Simulation General form of Snell’s law and critical angle n 1 : refractive index of medium 1 n 2 : refractive index of medium 2 θ 1 : angle of incidence θ 2 : angle of refraction 1 Conditions for total internal reflection Teaching notes Revisit Chapter 18 in Wave Motion if necessary. Endoscopy 2.3 (a) θ 1 < c (b) θ 1 = c (c) θ 1 > c (total internal reflection) Fig. 2.28 When light meets a boundary where n 1 > n 2 n 2 n 1 medium 2 medium 1 θ 1 θ 2 θ 1 θ 1 endoscopy 內窺鏡檢查 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.