13. The Endoscope There are two optical fibres One for light, to illuminate the inside of the patient One for a camera to send the images back to the doctor. Key hole surgery
14. Total Internal Reflection Normal line This has to be equal or greater than 90 degrees Sin 90 = 1 Sin θ c = n 2 /n 1 Θ c is the critical angle n 1 = refractive index of the glass (1.5) n 2 = refractive index of the air (1.001) Sin-1 (1.001 / 1.5) Critical angle = 41.8 n 1 Sin θ 1 = n 2
15. n = 2.41 Diamonds sparkle Calculate the critical angle for a diamond Sin θ c = n 2 /n 1
17. Refraction Questions 1. Explain why substances with a high refractive index like diamond, sparkle. 2. A pulse of white light is sent straight down a fibre optic cable 1 km long. The refractive index for blue light is 1.639 and for red light 1.621. What time interval will there be between the two components when t hey reach the far end? 3. Calculate the critical angle of an optical fibre: a) without cladding if the glass has a refractive index of 1.56. b) when cladding is added of n = 1.49 c) what advantage is this? n = C/V Sin θ c = n 2 /n 1 C = 3 x 10 8 m/s n air = 1.01 Speed = Distance/ time
18. 1) As any angle larger than critical is totally internally reflected, the smaller the critical angle the easier it is to get internal reflections which cause the sparkle. The speed of the blue light, V b = c/1.639 = 1.830 × 10 8 m s -1 The speed of the red, V r = c/1.621 = 1.851 × 10 8 m s -1 time = distance / speed time taken by the blue light = 1.0 × 10 3 / 1.830 × 10 8 = 5.46 μs time taken by the red = 1.0 × 10 3 / 1.851 × 10 8 = 5.40 μs the time lag is 0.06 μs. 3. a) (just glass) sin θ c = 1/n glass = 1/1.56 = 0.641 so C = 39.9 o glass and cladding sin θ c = 1.49/1.56 and C = 72.8 o .
19. Total Internal Reflection Normal line This has to be equal or greater than 90 degrees Sin 90 = 1 Sin θ c = n 2 /n 1
20. To do at home Read pages 188- 194 Answer question on page 208 Q1, Q2 (sketch the diagram)
