3 4 Refraction and TIR Total Internal Reflection

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: Title: Aims: 11 September 2021 49 -50 Refraction and Total Internal Reflection Understand total internal reflection as a consequence of refraction Textbook pages: Key vocab: Key concepts: critical angle When a ray of light is incident on the boundary between two media, some light is reflected and some light is refracted into the second media. � When the angle of incidence is greater than some critical angle, total internal reflection occurs and there is no refraction. � The conservation of energy is obeyed at all times. � Key equations: Progress in topic: 1

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Refraction Revision 2

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Refraction Revision 3

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Refraction and Snell’s Law �Shine rays of light into the flat face of a semicircular glass or perspex block. �Record various angles of incidence and refraction. �Ensure the mid-point on the semi-circular blocks edge is at the centre �Complete the table with your measurements, and calculate the sine of the angles you measure. (resourcefulphysics. org) 4

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Notes on Refraction �Occurs due to the slowing down of a wavefront as it enters more optically dense media. �Also, occurs due to the speeding up of a wavefront as it enters less optically dense media. �Wavefronts can be thought of like waves approaching a beach, and an analogy is lines of soldiers marching and crossing into different terrain. Those arriving (or leaving) more difficult material slow down (or speed up) first, thus bending the direction of travel of the column of soldiers. 5

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Quick Answers on refractive index n = c / cs = 3. 0 × 108 / 1. 9 × 108 = 1. 5789 = 1. 58 2. ns = c / n = 3. 0 × 108 / 2. 41 = 1. 2 × 108 m s-1 3. nair sin θi = nglass sin θr, so sin θr = nair sin θi / nglass Blue light: sin θr = 1. 0 sin 48º / 1. 639 = 0. 4534 therefore θr = 26. 963 = 27. 0º Red light: sin θr = 1. 0 sin 48º / 1. 621 = 0. 4584 therefore θr = 27. 287 = 27. 3º Difference in angle = 0. 3º 1. 6

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Total Internal Reflection �Shine rays of light into the curved face of a semicircular glass or perspex block. �Mark the centre of the straight edge with a fine permanent pen before you start. �Follow the instructions given, and ensure you answer the questions. (resourcefulphysics. org) 7

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Key ideas from the practical 1. X = angle of incidence. Y= angle of reflection. Z = angle of refraction. 3. As X increases, Y and Z also increase. Once Z reaches 90° (at which point X and Y will still be less than 90°), then the beam is no longer split but is totally internally reflected. You may observe colours in the beam as 90° is reached. 4. For Z = 90 O then X = C (the critical angle) so from Snell’s Law so O sin. 90 = 1 so 8

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Work example – calculating the critical angle Calculate the critical angle in water at an air boundary. The critical angle in a medium is related to the refractive index of the medium by 9

3. 4 Refraction and TIR 11 September 2021 Total Internal Reflection Textbook pages: 49 -50 Uses of Total Internal Reflection – Optical Fibres � An important application of total internal reflection is in fibre optics. � Light is shone along a thin glass fibre and as it hits the glass-air boundary at more than the critical angle it reflects along inside the fibre. � A beam of light travels through a bundle of fibres and as long as the angle of incidence with the walls of a fibre is great enough it will be reflected along the fibre as shown in Figure 1 � The fibres may be between 0. 01 mm and 0. 002 mm in diameter and may be arranged at the same relative positions at both ends of the light pipe so that a clear image may be seen through it. 10

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Uses of Total Internal Reflection – Optical Fibres � No cladding – multiple reflections at a fairly small angle. � The effect of cladding the fibres with another glass of slightly lower refractive index is shown in the following two diagrams. ncore sin θcore = ncladding sin θcladding so θcore = C when sin θcladding = 1. 0 11

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Uses of Total Internal Reflection – Optical Fibres � The cladding increases the critical angle between the two materials. The benefits of this are: (a) only those rays that are close to the axis of the fibre pass through (b) the inner fibre is protected from damage (c) the rays all travel roughly the same distance and so information fed in at one end arrives at the other only very slightly spread out in time (d) there are fewer reflections and the distance travelled is smaller than the multiple reflection case and so there is less energy loss and the time of transmission is shorter � Critical angle for glass air interface with n = 1. 55 = 41. 8 o � Critical angle between glass (n = 1. 55) and glass (n = 1. 45) = 69. 3 o 12

3. 4 Refraction and TIR Total Internal Reflection Textbook pages: 11 September 2021 49 -50 Uses of Optical Fibres 1. Illuminating models or road signs using only one bulb Advantages of fibre optics over copper wire 2. Endoscopy - seeing down inside a patient’s body 1. Cheap – glass is made from silica, the basic constituent of sand 3. Communications – sending 2. Light in weight – useful in information along a light beam. Useful for telephone, television, aircraft radio, computer networks, stereo links, control in aircraft 3. Light beam can carry a huge amount of information 4. Security fencing – very difficult to bypass 5. Fibre optic lamp 13