1. Light continued

2. Refraction of light is the bending of light as it goes from one optical medium to another
Less dense to more dense: bends towards normal
More dense to less dense: bends away from normal

3. A medium is

4. Index of Refraction

5. Incident ray i r
Refracted ray
Glass block

6. The Laws of Refraction of Light
1. The incident ray, the normal and the refracted ray all lie in the same plane
where n is a constant
This is called Snell’s Law

8. When light travels from a rarer to a denser medium it is refracted towards the normal e.g. air to water

9. when light travels from a denser to a rarer medium it is refracted away from the normal e.g. glass to air

11. Experiment to Verify Snell’s Law and determine the refractive index of glass

12. i 1 2 r
Block of glass 3 4

13. Experiment to Verify Snell’s Law and determine the refractive index of glass
Method
Outline the glass block on paper
Stick pins 1 and 2 in the paper in front of the block
Stick pins 3 and 4 in line with the images of 1 and 2, as seen through the block

14. 4. Remove the pins, join the pinholes and draw the normal
5. Measure angles i and r
6. Repeat for different angles of incidence
7. Draw a graph of sin i vs. sin r
8. The slope of the graph is the refractive index of glass

15. Result i r
Sin i
Sin r
Sin i/Sin r

16. Sin i
Sin r

18. Slope of graph =
Thus the refractive index of glass is ____

19. Conclusion
The graph is a straight line graph through the origin, thus verifying Snell’s Law
The slope of the graph gave the refractive index of glass

20. Real and Apparent Depth

21. A swimming pool appears to be less deep than it actually is, due to refraction at the surface of the water
We can calculate the refractive index of a liquid by using
n =

23. Refractive index
n =

25. Question 1
When light passes from air into a liquid, the angle of incidence and refraction are 57˚and 30˚ respectively. Calculate the refractive index of the liquid.
Answer
Formula:

26. Question 2
A ray of light is incident at 35˚ on (a) a glass surface and (b) a water surface. Calculate the angle of refraction given that,
ng=1.52 nw=1.33
Answer
Formula:

27. Textbook Examples
Questions 1-3 on page 31

29. Total Internal Reflection
This may occur when light goes from a denser to a less dense medium
As i is increased so is r
Eventually r = 90˚
At this point i has reached the ‘critical angle’

33. r = 90˚
i = critical angle

34. Total Internal Reflection!!!

35. It is reflected back into the first medium
If i is increased beyond the critical angle, the ray does not enter the second medium
It is reflected back into the first medium
We can also find the refractive index of a material using
n =
C = critical angle

36. Example The critical angle of glass is 41.81˚
Find the refractive index of glass
n =
n = 1/0.666
n = 1.5

37. Refractive index
n =

38. Example The refractive index of glass is 1.5
This value is for a ray of light travelling from air into glass
So ang = 1.5 =
Or gna = =

39. Applications of Total Internal Reflection
Periscopes (using a prism)
Diamonds and bicycle reflectors
Optical fibres – in telecommunications and by doctors

40. Prisms
Prisms are considered to be better than mirrors for reflecting light. This is because they use total internal reflection.
This is the reason prisms are used in periscopes and binoculars.

42. Optical Fibres
Total Internal Reflection has given rise to the use of optical fibres.
The signal is fired at an angle greater than the critical angle so that it is reflected all the way along the fibre.
The light is therefore trapped within the fibre. This technology is used to transmit information over long distances.

43. Endoscopes
These are used in medicine. They contain two bundles of optical fibres, one to carry light to the organ being viewed and the other to carry the image of the organ back to the operator.

44. Optic Fibre Glass cladding of low refractive index
Glass core of high refractive index

45. Glass of high refractive index
Glass of low refractive index N N
Normal

47. Remember that rays are path-reversible
AIR
GLASS A B

48. Mirages
Mirages are caused by the refraction of light in air due to temperature variations

50. SKY

51. Convex lens (converging)
LENSES
Made of transparent material, usually glass
Convex lens (converging)
Thick in centre and thin at edges

52. Concave lens (diverging)
Thin in the centre and thick at the edges

54. Ray diagrams for lenses
1. Ray incident parallel to principal axis is reflected out through focus
2. Ray incident through focus is reflected out parallel to axis
3. Ray incident through optic centre continues in straight line

55. RAY DIAGRAMS FOR CONVEX LENSES
Optic centre

56. RAY DIAGRAMS FOR CONCAVE LENSES
Optic centre

57. Geometric Optics 2.03

58. The image in a concave lens is always virtual, erect, and diminished
Concave lens always has negative f and v

59. YouTube - 3D Medical Animation: LASIK Eye Surgery for Nearsightedness

60. Lens formulae
m =

61. Example
An object is placed 10cm away from a convex lens of focal length 12 cm. Calculate the nature, position and magnification of the image.

62. f = 12
u = 10
v = ?
v = -60cm

63. m =
m = = 6
So the image is formed 60cm from the lens
It is virtual
Magnification is 6

64. Example 2
An object is placed 40cm from a concave lens of focal length 50cm. Find the position, nature and magnification of the image.

65. u = 40cm
f = -50cm
v = ??
v = cm

66. m =
m = = 0.56
So the image is formed 22.2cm from the lens
It is virtual
Magnification is 0.56

67. Experiment to Measure the Focal Length of a Convex Lens
Pin
Plane mirror

71. Experiment to Measure the Focal Length of a Convex Lens
Method
A rough estimate of the focal length may first be obtained by focusing the image of a distant object on a sheet of paper
Set up the apparatus as in the diagram
Move the pin in and out until there is no parallax between the pin and its image
Measure the distance from the pin to the centre of the lens. This is the focal length

72. Result
The distance from the pin to the centre of the lens was ______
Conclusion
The focal length of the lens is ______

73. Experiment to Measure the Focal Length of a Concave Lens
Convex lens
Pin F
Plane mirror

74. Method Set up the apparatus as in the diagram
The focal length of the convex lens is first measured
The concave lens is combined with the convex lens (of shorter focal length), and so overall it behaves as a convex lens
Move the pin in and out until there is no parallax between the pin and its image
Measure the distance from the pin to the centre of the lens. This is the focal length of the combination

75. Results The focal length of the convex lens is 22cm
The focal length of the combination is 51cm.
The focal length of the concave lens can be calculated from the formula
Where F = focal length of combination
f1 = focal length of convex lens
f2 = focal length of concave lens

76. F = 51cm
f1 = 22
f2 = ??

77. Conclusion
The focal length of the concave lens is 38.69cm