6 total internal reflection


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6 total internal reflection

  1. 1. Total Internal Reflection
  2. 2. glass Reflection and refraction • Mirror reflects light • Transparent medium refracts light mirror
  3. 3. Which angle is larger? • A light ray is emerging from glass to air. Which angle is larger, in glass or in the air ? glass normal Or this? This? AIR
  4. 4. glass Which angle is larger? • A light ray is emerging from glass to air. Which angle is larger, in glass or in the air ? normal Or this? This? Next AIR
  5. 5. Critical Angle • For glass-air interface, the angle of refraction in air > incident angle glass AIR normal Incident angle Angle of refraction Partial reflection
  6. 6. Which angle first? If the angle of incidence increases, which angle will reach 90o first, the angle of incidence or the angle of refraction? A Angle of incidence (from glass) B Angle of refraction (in the air) Next
  7. 7. AIR Critical angle (2) • When we increase the angle of incidence (i), the angle of refraction (r) increases until ...... AIR glassglass AIR glass AIR glass i r normal
  8. 8. AIR glass Critical angle (3) • when the angle of refraction is 90o , the incident angle is called the critical angle (C). AIR glass r90o normal iC
  9. 9. sin 90o sin C Critical angle (4) • For glass-air interface, the critical angle is 42o . • At critical angle, angle of refraction is 90o , so, by the laws of refraction, n = sin r sin i = 1 n x sin C = 1 sin C = 1 n
  10. 10. What is the critical angle... … for water-air interface? The refractive index of water is 1.33 sin C = 1 n 1 ÷ 1.33 = 0.75 sin-1 0.75 = 48.8o => C = 48.75o
  11. 11. For glass, the refractive index is 1.5; C = sin-1 (1÷1.5) Critical angle = 41.81o Critical angle (5) sin C = 1 n For water, the refractive index is 1.33; C = sin-1 (1÷1.33) Critical angle = 48.75o 1 n C = sin-1 ( )
  12. 12. What if ... … the angle of incidence (from glass) is larger than the critical angle? A The angle of refraction will become smaller again. B There will be no refracted ray. C The light ray will be reflected the same way back. Next
  13. 13. Total internal reflection (1) • When the light ray (from water) is emerging at the critical angle, the refracted ray will be along the interface. water AIR alongalong
  14. 14. Total internal reflection (2) • When the incident angle is larger than the critical angle, total internal reflection will occur (at the interface). water AIR Next
  15. 15. Total internal reflection (3) • View under water!! water AIR
  16. 16. Total internal reflection (4) • Fish-eye view You see total internal reflection here You see the sky here through refraction You see water waves here total internal reflection here total internal reflection here sky waves waves
  17. 17. Can you explain the ... • fish-eye view by drawing light rays? water Yes
  18. 18. Fish eye view (1) water Central hole, shows the squeezed view of the sky totally internally reflected ray
  19. 19. Fish eye view (2) • When a diver or fish looks upward under water, part of the water surface looks like a mirror, except the central hole. • So, the light entering the water from above the surface is squeezed into a cone of angle 2C 2C water What is C ? C is critical angle
  20. 20. Revisit the fish eye view • Do you remember which part has refraction and which part has total internal reflection? total internal reflection here sky(refraction here)
  21. 21. What did you learn? • Critical angle • Total internal reflection occurs when a light ray tries to escape from water (or glass) to air at an angle of incidence larger than the critical angle at the interface. • Fish eye view is an example of total internal reflection and refraction. 1 n C = sin-1 ( )