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# LIGHT

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1. LIGHT Everything written in black has to go into your notebook Everything written in blue should already be in there

2. WHAT IS LIGHT? • Light is a form of energy that travels away from the source producing it at a speed of 3 x 108 m s-1

3. Transparent: allows light to pass through it, and can see clearly through it e.g. glass • Translucent: allows light to pass through it, but cannot see clearly through it e.g. frosted glass • Opaque: does not allow light to pass through it e.g. aluminium

4. Light Travels in Straight Lines Light travels in straight lines. This can be seen in the following examples • Laser • Beam of light from a searchlight It can also be shown using pieces of cardboard with a small hole in the middle and a length of thread

5. A plane mirror is a flat mirror

6. Plane Mirror (diagram on page 1) Normal Incident ray Reflected ray Angle ofincidence Angle ofreflection i r Plane Mirror

7. LAWS OF REFLECTION OF LIGHT • 1. The incident ray, the normal and the reflected ray all lie in the same plane • 2. The angle of incidence is equal to the angle of reflection (i = r)

8. HOW IS AN IMAGE FORMED IN A PLANE MIRROR (diagram page 2)

9. Properties of an image in a plane mirror The image is: • Laterally inverted • E.g. your right hand appears as a left hand • The “ambulance” sign • Erect • Virtual • Same size as object

10. Uses of Plane Mirrors • Make up mirror • The periscope

11. Diagram page 3

12. A virtual image cannot be formed on a screen • A real image can be formed on a screen

13. Experiment to prove the angle of incidence equals the angle of reflection (page 26)

14. Diagram on page 26 Plane mirror Sheet of paper r i Pins

15. Reflection is the bouncing of light off an object

16. Experiment to prove the angle of incidence equals the angle of reflection (written up in homework copy)

17. Diagram (in homework copy) Finder pin Plane mirror Object pin O M I

18. The following goes in your homework copy • Method • Set up the apparatus as in the diagram • Move the finder pin in and out behind the mirror until there is no parallax between the object and its image in the mirror

19. 3. Measure the distance from the object to the mirror (OM), and the distance from the mirror to the image pin (MI) Result OM and MI are equal Conclusion The image is as far behind the mirror as the object is in front of it

20. Spherical Mirrors (page 4) CONVEX CONCAVE

21. The line from the centre of curvature to the pole is called the principal axis

22. Rules for Ray Diagrams for Concave Mirror • 1. A ray travelling parallel to the principal axis is reflected through the focus • 2. A ray travelling through the focus is reflected parallel to the principal axis • 3. For a ray which strikes the pole, angle i will be equal to angle r

23. Top of page 5 • “In parallel, out through the focus” • “In through the focus, out parallel”

24. Uses of concave mirrors • Spotlights • Reflectors in car headlights • Shaving and make-up mirrors

25. Uses of convex mirrors • Shops (to deter shoplifters) • Buses • Dangerous bends in roads • They give a wide field of view

26. The Mirror Formulae u = distance from object to mirror v = distance from image to mirror f = focal length

27. Example 2 • When an object is placed 16 cm in front of a concave mirror of focal length 8 cm, an image is formed. Find the distance of the image from the mirror and say whether it is real or virtual.

28. v = 16 cm

29. It is a real image since the object is outside f

30. Magnification • m = • m =

31. Example 3 (HL) • An object is placed 20 cm from a concave mirror of focal length 25 cm. Find the position, magnification and nature of the image.

32. v = 100 cm

33. It is a virtual image since the object is inside f

34. m = • m = • m = 5

35. Example 4 (HL) • A concave mirror of focal length 10 cm forms an erect image four times the size of the object. Calculate the object distance and its nature.

36. u = 7.5 cm

37. It is a virtual image since the object is inside f

38. CROSS THREADS RAY BOX CONCAVE MIRROR SCREEN Diagram page 30

39. Light (2) Refraction and Lenses

40. Refraction of light is the bending of light as it goes from one optical medium to another • A medium is a substance; e.g. glass, air etc.

41. Incident ray i r Refracted ray Glass block

42. (Page 12, under diagram) • Less dense to more dense: bends towards normal • More dense to less dense: bends away from normal

43. The Laws of Refraction of Light • 1. The incident ray, the normal and the refracted ray all lie in the same plane • 2. where n is a constant • This is called Snell’s Law

44. Experiment to Verify Snell’s Law and determine the refractive index of glass (diagram page 27) Pins Glass Block Sheet of paper

45. Enter the following results at the top of page 28, and draw the corresponding graph underneath

46. Your graph in page 28 should look like this Sin i Sin r

47. Real and Apparent Depth (page 12) • 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 =

48. Critical angle • The critical angle is the angle of incidence in the denser medium when the angle of refraction is 90˚

49. Total Internal Reflection • This occurs when the angle of incidence in the denser medium exceed the critical angle • The ray of light is refracted away from the normal • As i is increased so is r • Eventually r = 90˚ • At this point i has reached the ‘critical angle’ • If i is increased beyond the critical angle, the ray does not enter the second medium • It is reflected back into the first medium