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Total internal reflection

Total internal reflection. dim image. only very small amount of light is reflected by the glass. the glass surface is not smooth enough. the environment outside is not dark enough. light ray should undergo refraction instead of reflection.

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Total internal reflection

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  1. Total internal reflection

  2. dim image only very small amount of light is reflected by the glass. the glass surface is not smooth enough. the environment outside is not dark enough. light ray should undergo refraction instead of reflection. The picture shows the image of a boy formed by a glass. 

  3. Introduction The inside surface of water or a glass block can act like a mirror.

  4. Total internal reflection Simulation Video

  5. 1 Internal reflection A light ray hits the inside face of a semicircular block as follows. air glass What will happen?

  6. 1 Internal reflection For a small angle of incidence • The incident ray splits into 2 rays. • angle of refraction < 90o refracted ray air glass incident ray reflected ray

  7. 1 Internal reflection • as angle of incidence , angle of refraction  • eventually, angle of refraction = 90 • angle of incidence = critical angle C refracted ray air glass C C incident ray reflected ray

  8. 1 Internal reflection • angle of incidence continues to , norefracted ray air glass incident ray reflected ray

  9. 1 Internal reflection • Therefore, if angle of incidence > C, the light ray is totally reflected inside. air glass incident ray reflected ray • This is called total internal reflection.

  10. 1 Internal reflection Video

  11. Critical angle and refractive index 1 sin 90o sin C sin C C C 1 ( ) n refracted ray air glass incident ray reflected ray n = C = sin1  n = or

  12. Critical angle and refractive index Refractive index Critical angle Medium Glass 1.50–1.70 30–42 Water 49 1.33 Perspex 1.5 42 Diamond 2.42 24 • critical angles of different materials

  13. Examples of total internal Reflection a Prisms in optical instruments • plane mirror = glass + silvered surface glass sheet silvered surface forms the image

  14. I1 I2 I3 O object • multiple reflection inside the glass • multiple images formed • nuisance in optical instruments

  15. 45 45 • If light rays strike the inside face at an angle > 42, glass prism behaves like a perfect mirror. 45 45 45 45

  16. Prisms are used as mirrors in periscopes and binoculars.

  17. film mirror • In single-lens reflex camera A five-sided ‘pentaprism’ reflects light from the mirror into the eye.

  18. 3 Examples of total internal reflection b Mirage In a desert. On a dry road.

  19. 3 Examples of total internal reflection b Mirage • On a hot day, you may see a pool of water at a distance in a desert. Hurray! Sorry! What you see is just aMIRAGE.

  20. It happens due to total internal reflections. Image of the sky layers of air near the ground –hot – less dense than upper air –lower refractive indices

  21. Image of the sky • light refracted more and more towards the horizontal

  22. When light meets a layer of air near the ground at an angle greater than C, Image of the sky Total internal reflections occurs.

  23. b Mirage Video Video

  24. 3 Examples of total internal reflection c Light guides • Light can pass through a curved perspex rod! • The rays bounce inside the rod. • None of them is lost to the air from the sides.

  25. bundle of fibres glass fibre Applications 1 optical fibre cable • made up of a bundle of glass fibres 2 as a ‘light guide’ for the doctor to see inside the human body

  26. c Light guides Video Video

  27. Example 5 E 30 O 18 A ray of light travelling in the direction EO in air enters a rectangular block. angle of incidence = 30 angle of refraction = 18 (a) Find the refractive index n of the block. (b) Find the critical angle C of the ray for the block.

  28. Example 5 sin 30 sin 18  n = =1.62 1 1.62 (b) C = sin1 = sin1 1 n (a) By Snell’s law, n sin 18 = 1  sin 30 = 38.1

  29. Example 5 C D 30 B A (c) If the ray is incident on surface BC, from which surface and at what angle will the ray leave the block?

  30. Example 5 C D 30 32.3 57.7 32.3 B A (c) The ray comes out from surface AD. The outgoing angle from normal is 60. 60

  31. Q1 Which of the following… A B C D Which of the following angles is the critical angle of glass?

  32. Q2 A horizontal light ray is… A B 45 C A horizontal light ray hits a prism as shown. What happens to the light ray?

  33. Q3 If the breaker is 12 cm tall... A water small stone If the beaker is 12 cm tall and 8 cm wide, Can we always see the small stone below water from side A? (Given: Refractive index of water = 1.33)

  34. Q3 If the breaker is 12 cm tall... A water small stone Critical angle of water 48.8 0.752 1.33

  35. Q3 If the breaker is 12 cm tall... A water small stone For the light ray from the stone reaching side A, Maximum angle of incidence on side A 12 56.3 8

  36. Q3 If the breaker is 12 cm tall... A water small stone greater Since is ________ than the critical angle, ______________________ occurs and we _________ always see the stone form side A. total internal reflection cannot

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