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# [Unit 12.2] refraction of light

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### [Unit 12.2] refraction of light

1. 1. Lig ht Refraction of
2. 2. plane  normal  normal center Normal :
3. 3. Angle of incidence, i Angle of refraction, r incident ray Ray emerges parallel to incident ray Refracted ray Refraction of light: The change in the direction, or bending of light when it travels from one medium to another is known as refraction .
4. 4. <ul><li>When light travels from a less dense medium to a denser medium the speed of light decreases and the light ray is refracted towards to the normal . </li></ul>
5. 5. i r i > r since D air < D glass  a  g  a >  g normal Air Glass When light travels from a less dense medium to a denser medium the speed (or wavelength) of light decreases and the light ray is refracted towards to the normal. Refraction of light:
6. 6. Laws of refraction: <ul><li>The incident ray, refracted ray and the normal at the point of incidence all lie on the same plane. </li></ul><ul><li>Snell’s law: </li></ul><ul><li>When light travels from a first medium to a second medium, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant called the refractive index. </li></ul>(continue in next slide)
7. 7. Refractive index 1.0 Air 1.3 Ice 1.33 Water 1.5 Perspex About 1.5 Glass 2.4 Diamond Refractive Index Medium
8. 8. Refractive index <ul><li>The greater the refractive index n, the slower is the speed of light in that medium and the light bends more in that medium </li></ul><ul><li>The refractive index cannot be less than 1. </li></ul>
9. 9. Important Formulae: <ul><li>n = sin i/ sin r </li></ul><ul><li>n= (speed of light in the first medium)/(speed of light in the second medium) </li></ul><ul><li>where n is the refractive index of the second medium with respect to the first medium (vacuum). </li></ul>
10. 10. Apparent depth Real depth Fish image of fish Air Water Refractive index of water = (Real depth / Apparent depth)
11. 11. Example: <ul><li>Find the refractive index of glass when light travels from air to glass at the angle of incidence of 45 o and an angle of refraction of 28 0 . </li></ul>Solution: Using n= sin i/ sin r, = sin 45 0 / sin 28 0 =1.5 The refractive index of glass is 1.5. 45 0 28 0
12. 12. <ul><li>When light travels from a less dense to a denser medium, the speed of light decreases and the light ray is refracted towards the normal. </li></ul>
13. 13. Example: <ul><li>The refractive index of water is 1.33. A girl views the bottom of a pond of depth 1.5m directly from above. What is the apparent depth of the pond? </li></ul>Solution: refractive index of water = real depth / apparent depth 1.33 = 1.5 / d d = 1.5 / 1.33 = 1.13 m
14. 14. Reversibility of light in reflection Two fig. seen alike, Why ? Explain ! speed v 1 speed v 2 speed v 2 speed v 1  1  1  2  2 n 1 n 1 n 2 n 2
15. 15. air water air water air water i i i = c, critical angle i < c, critical angle i > c i Case 1 Case 2 Case 3  Reflection When light ray traveled from denser material to less dense material (say, from water to air), and angle of incidence, i < c, the critical angle, most of the light ray will be refracted to the less dense medium and part of the light will be reflected. (case 1)
16. 16. air water air water air water i i i = c, critical angle i < c, critical angle i > c i Case 1 Case 2 Case 3  When light ray traveled from denser material to less dense material (say, from water to air), if the angle of refraction equal to 90 0 , then its angle of incidence at this instant is defined as critical angle of the denser material. (case 2) angle i = angle c (if angle r = 90 0 ) Critical Angle
17. 17. air water air water air water i i i = c, critical angle i < c, critical angle i > c i Case 1 Case 2 Case 3  Total Internal Reflection When light ray traveled from denser material to less dense material (say, from water to air), and i f the angle of incidence, i > c, the critical angle, all the light ray will reflected internally. This is called total internal reflection. (case 3)
18. 18. Critical Angle <ul><li>since </li></ul><ul><ul><li>refracted index </li></ul></ul><ul><ul><li>= (sin x) / (sin y) </li></ul></ul><ul><ul><li>therefore </li></ul></ul><ul><ul><li>n = (sin x)/ (sin y) </li></ul></ul><ul><ul><li>when x = 90 0 and </li></ul></ul><ul><ul><li>y = c, the critical angle </li></ul></ul><ul><ul><li>Hence sin c = 1 / n </li></ul></ul>normal Denser medium Less dense medium c =y x
19. 19. high density low density Optical Fibre ( Group Discussion) light ray
20. 20. Mirages
21. 21. GCE O-Level Past Examination Paper Science (Physics)
22. 22. C O’ level Physics Nov 1995 12. The diagram shows a ray of light moving from air into plastic. What is the refractive index of the plastic ? A 0.71 B 1.22 C 1.41 D 1.50
23. 23. GCE O Nov 1996 10. A ray of light travels from air into glass. The angle of incidence is i and the angle of refraction is r. How is the refractive index of glass calculated ? A i / r B sin i / sin r C sin ( i/r ) D sin i x sin r B
24. 24. Nov 1991 11. Which line shows the path a ray of light would take through a glass block? B Hint: Incident ray parallel to ray B
25. 25. GCE O Nov 1996 11. A ray of red light travelling in glass strike the glass-air boundary. Some light is reflected and some is refracted. Which diagram shows the paths of the ray ? A
26. 26. Nov 1990 7. The bottom surface of a glass block is silvered to act as a mirror. Which diagram best represents the path of a light ray that enters this block through the top surface? E
27. 27. GCE O Nov 1997 10. The bottom surface of a glass black is silvered to act as a mirror. Which diagram could represent the path of a light ray which enters this block through the top surface ? D
28. 28. GCE O Nov 1994 12. Which diagram shows the path of a ray of light through a glass block ? D
29. 29. November 1989 9. A prism is made from glass with a critical angle less than 45 o . Which one of the following diagrams, showing light passing through the prism, is incorrect? C
30. 30. November 1989 11. When a narrow beam of white light passes through a prism, it produces a spectrum. Which one of the following diagrams correctly shows the refraction and dispersion produced by the prism? E Note: not in 1999 syllabus
31. 31. Nov 1990 8. Which diagram correctly shows how a prism can be used to split white light into a spectrum? (Only the colours at the ends of the spectrum are marked on the diagrams.) E
32. 32. 10.(a) Draw a diagram to show a ray of light passing through from air to glass. Label the angle of incidence i and the angle of refraction r. Define refractive index for the glass. [3] GCE ‘O’ LEVEL Nov 1995 i r air glass i > r The refractive index of the glass is equal to the ratio of sin i and sin r. Thus, n = sin i / sin r (continue on next slide)
33. 33. 10.(b) Describe an experiment you would carry out to determine the refractive index of glass, given a rectangle glass block. State the measurements you would make and show how you would use them to calculate the result. [4] (Cont. …) Q. 10 Nov 1995 i <ul><li>label the rectangular </li></ul><ul><li>glass block as shown in </li></ul><ul><li>the figure. </li></ul> P 1  P 2 <ul><li>Choose angle of incidence </li></ul><ul><li>i (=30 0 ) and draw incident </li></ul><ul><li>ray as shown. </li></ul><ul><li>Put two pins P 1 and P 2 on the incident ray as </li></ul><ul><li>shown in the fig. </li></ul>(continue on next slide)
34. 34. i  P 1  P 2 (Cont. …) Q. 10 Nov 1995 r  <ul><li>10.(b) </li></ul><ul><li>From the opposite side , observe the two images of </li></ul><ul><li>pins P 1 and P 2 and then insert another two pins P 3 </li></ul><ul><li>and P 4 , so that the images and these two pins lie in </li></ul><ul><li>same straight line as shown </li></ul><ul><li>in the fig. </li></ul> P 3 P 4 <ul><li>Hence emergent ray is </li></ul><ul><li>obtained, and angle of </li></ul><ul><li>refraction also obtained. </li></ul><ul><li>Repeat the above mentioned </li></ul><ul><li>steps with 4 different set of i and then obtained another 4 sets of r. </li></ul>(continue on next slide)
35. 35. (Cont. …) Q. 10 Nov 1995 <ul><li>10. (b) </li></ul><ul><li>List the values of i and r in the following table. </li></ul>angle of incidence, i angle of refraction, r <ul><li>Hence, calculate 5 sets of the ratio sin i / sin r. </li></ul><ul><li>Then the average of the ratios that determined is </li></ul><ul><li>the refractive index of the glass. </li></ul>
36. 36. November 1992 3. The diagram shows a ray of light passing thorough a glass block ABCD. (a) Calculate the refractive index of the glass, showing your working. 15 0 25 0 A B C D n = sin i / sin r = sin 25 0 / sin 15 0 = 1.63 (continue on next slide)
37. 37. 3(b) light passing thorough the block is partially reflected at two places. Draw, on the diagram both the reflected rays. [3] 25 0 15 0 25 0 (Cont. …) Q. 3 November 1992
38. 38. Nov 1990 4. The diagram below shows a ray of red light approaching a point P on the boundary between glass and air. a) Which medium, A or B, is the glass? [1] b) What is the size of the angle  ? [1] Since  > 30 0 , therefore median B is the glass angle  = 30 0 (continue in next slide)
39. 39. 4(c) The incident beam was then changed to white light. What effect would this have on the light rays (i) PX, (ii) PY? [2] (Cont. …) Q. 43 Nov 1990 White light Since white light has higher frequency than red light, therefore, (i) PX remains unchanged . (ii) PY will deflect slightly away from normal.