2. Protons for Breakfast Week 2 Light November 2012

3. In the event of an alarm sounding…

4. Toilets…

5. Parents and children…

6. Last Week’s talk The scale and size of the Universe Its very big, but full of very small things The electric force It dominates physical phenomena on our scale. How the force works Electric particles Electric field

7. This Week’s talk Light Waves in the Electric field I want you believe that light is a wave! Frequency What is frequency? Relationship between light and atoms All the light you see comes ‘fresh’ from an atom Michael – are you going to tell them that this is the most intellectually demanding week?

8. Electromagnetic waves Electricity Heat How it all fits together… Atoms

9. Looking again at what we saw last week…

10. Odd phenomena… A balloon and a piece of paper

11. Lets take a look at some odd phenomena… A balloon and an electroscope

12. Van de Graaff

14. The electrical nature of matter Electric charge is a fundamental property of electrons and protons. Two types of charge (+ and -) If particles have the same sign of electric charge they repel If particles have different signs of electric charge they attract The forces (attractive or repulsive) get weaker as the particles get further apart.

15. How do charges affect other charges? It’s a three-step process Particles with electric charge affect the field The effect propagates through the field The field affects other particles with electric charge …but the steps happen very quickly

16. How do charged particles interact? It’s a three-step process… Particle with electric charge Particle with electric charge Interact by means of an electric field …but the steps happen very quickly

17. How do we describe the world?

18. The nature of interactions (1) Analogy with water level and water waves

19. Now let’s move on…

20. Electric Gherkin

21. What happens when you electrocute a gherkin? The Gherkinator Button of death ???????

22. What is light ? A Question

23. Lets take a look at some odd phenomena… A balloon and an electroscope

24. Lets take a look at some odd phenomena… A balloon and an electroscope Wiggling the balloon… Causes the electroscope to wiggle

25. Lets take a look at some odd phenomena… The balloon is a source of electric waves (technically electromagnetic) waves. The waving electroscope is a detector of electric waves

26. Frequency

27. 1 oscillation per second is called 1 hertz

28. Frequency… oscillations per secondis called a… 1000 (a thousand) (10 3 ) kilohertz (kHz) 1000000 (a million) (10 6 ) megahertz (MHz) 1000000000 (a billion) (10 9 ) gigahertz (GHz) 1000000000000 (a trillion) (10 12 ) terahertz (THz) 1000000000000000 (a million billion) (10 15 ) petahertz (PHz)

29. Did you do your homework? What was the frequency your favourite radio station? Radio 4 ‘long wave’ –198 kHz ‘Medium wave’ –540 kHz to 1600 kHz ‘FM’ stations –88 MHz to108 MHz Digital Radio –217 MHz to 230 MHz

30. Electromagnetic waves (1) Electromagnetic waves can be generated with a vast range of frequencies The complete range is called the electromagnetic spectrum We give different names to different frequencies of electromagnetic waves Different frequencies require quite different types of equipment to generate to detect

31. 1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 910 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18 10 19 10 20 10 21 10 22 Radio & TV Infra Red Microwaves Gamma- Rays X-Rays Ultra Violet Frequency (Hertz) 1000 THz (Blue) 400 THz (Red) Electromagnetic spectrum

32. 1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 910 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18 10 19 10 20 10 21 10 22 Radio & TV Infra Red Microwaves Gamma- Rays X-Rays Ultra Violet Non-ionising Radiation (generally not so bad) Ionising Radiation (generally bad) Frequency (Hertz) Electromagnetic spectrum

33. Michael: don’t forget the Jelly Baby Wave Machine! Jelly Baby Wave Machine

34. the wave moves from one place to another, the jelly babies just move up and down Jelly Baby Wave Machine

35. Is light really a wave in the electric field?

36. How can we prove that light is a wave? Historically this ‘proof’ was obtained by Thomas Young He performed a famous ‘double slit’ experiment We will perform a similar experiment.

37. Young’s experiment A double slit This is how Young conceived of the experiment

38. Our Experiment A laser gives light with just a single frequency What would we expect to see if we shine it at a screen? LASER Screen

39. Our Experiment We will place a thin wire in the centre of the laser beam What would we expect to see if we shine it at a screen? LASER Thin wire suspended in light beam Screen ?

40. Our Experiment What do we actually see? LASER Thin wire suspended in light beam Screen

41. This can only be explained if light is a wave

42. Interference

44. Overlapping Semicircles (1) Wire Screen

45. Diffraction Patterns The pattern seen on the screen depends on The wavelength of the light The thickness of the wire Seeing these bright and dark bands establishes beyond doubt that light has a wave nature.

46. Overlapping crop circles Images Steve Alexander Copyright 2004

47. Interference Simulation Interference simulation

48. Now with Red Light What happens if we do the experiment with red light? LASER Thin wire suspended in light beam Screen

49. Diffraction Patterns

50. Light is a wave Wavelength is just less than one thousandth of a millimetre

51. What is a Diffraction Grating? We can exploit the diffraction of light through a grating Different frequencies of light have different wavelengths A diffraction ‘grating’ separates light into its different frequencies we can look at the ‘structure’ of light. We perceive different frequencies of light to have different colours

52. Diffraction Grating An array of fine lines…

53. Spectroscopic glasses What do you see?

54. Break Left-Hand Side 15 minutes to look at some lights 15 minutes to hear Andrew talk about Colour Perception Right-Hand Side 15 minutes to hear Andrew talk about Colour Perception 15 minutes to look at some lights

55. Photo credit http://home.comcast.net/~mcculloch-brown/astro/spectrostar.html What I hope you saw! Filament Lamp 700 nm 700 nanometres 0.7 thousandths of a millimetre 400 nm 400 nanometres 0.4 thousandths of a millimetre Fluorescent Lamp

56. Afterbreak summary Light is a wave in the electric field Frequency 400 THz (Red) 1000 THz (Blue) Wavelength 0.7 thousandths of a mm (Red) 0.4 thousandths of a mm (Blue) Speed 300000 kilometres per second 186000 miles per second

57. Afterbreak Questions 1.Why are some spectra made of discrete lines? 2.Why are some spectra continuous? 3.What about light from molecules rather than atoms? 4.What makes an object coloured?

58. All light comes ‘fresh’ from atoms

59. Afterbreak Questions 1.Why are some spectra made of discrete lines? Atoms are unconstrained: resonance 2.Why are some spectra continuous? Atoms are constrained 3.What about light from molecules rather than atoms? Good Question! 4.What makes an object coloured? As Andrew showed, its quite complicated!

60. Lets remind ourselves about atoms (1) The internal structure of atoms Electrons ‘orbit’ around the outside of an atom very light possess a property called electric charge Nucleus occupies the centre very tiny and very heavy protons have a property called electric charge neutrons have no electric charge

61. Lets remind ourselves about atoms (2) Nuclei (+) attract electrons (-) until the atom as a whole is neutral The electrons repel each other They try to get as far away from each other as they can, a and as near to the nucleus as they can Electrons Electrons possess 1 unit of negative charge Nucleus protons possess 1 unit of positive charge neutrons have no electric charge

62. How do we make light? We make light by ‘hitting’ an atom: hard ‘Strike’ it with an other atom ‘Strike’ it with an electron To make a wave at 1 petahertz (10 15 hertz) we need : Enormous forces Very light particles Enormous forces come the electric forces within an atom Very light particles are electrons within an atom

63. 1. Discrete Spectra

64. Light from atoms… If an atom or molecule is ‘unconstrained’ then When it is hit, it ‘rings’ like a bell Atoms ‘ring’ at their natural frequency: resonance Each type of atom vibrates in a characteristic manner.

65. Light from atoms We know about every type of atom that can exist. And we know its spectrum…And we know its spectrum

66. Light from atoms We know about every type of atom that can exist. And we know its spectrum… Hydrogen Helium Lithium Carbon Nitrogen Oxygen http://laserstars.org/data/elements/ Sodium Xenon Neon ‘Atomic Fingerprints’

67. The light from the gherkin came from Sodium atoms Light from atoms The Gherkinator Button of death

68. Light from atoms (6) The Gherkinator In my office…

69. Light from atoms The Gherkinator The gherkin has a discrete spectral line at around 589 nm This indicates the presence of sodium atoms

70. 2. Continuous Spectra

71. If an atom or molecule is ‘constrained’ then it cannot ‘ring’ clearly. The light which emerges has a mixture of all possible frequencies The balance of colours in the spectrum depends on how fast the atoms are jiggling – i.e. on temperature. Light from atoms in solids (1)

72. Light from atoms in solids (2) The filament of a light bulb is heated to ~2500 °C to make it give off ‘white’ light When something is at about 800 celsius: its red hot When its colder, it gives off infra-red light. We can’t ‘see’ this light but we can detect it.

73. 1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 910 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18 10 19 10 20 10 21 10 22 Radio & TV Infra Red Microwaves Gamma- Rays X-Rays Ultra Violet Frequency (Hertz) Electromagnetic spectrum Cold Hot

74. 3. Infra Red Light

75. H Atoms & Molecules H2H2 N N A molecule is a collection of atoms stuck together electrically. H H 0 H20H20 H N2N2

76. What happens if you knock a molecule? If a molecule is hit, the atoms within the molecule vibrate. Atoms are thousands of times heavier than electrons So they ‘ring’ with a much lower frequencies. The light given off is in the infra red range of the spectrum. H20H20

77. Some molecules vibrating Different types of molecular jiggling occur at different frequencies

78. Colour Perception

79. What makes an object coloured? Yellow Blue

80. What is colour? When we say ‘That object is ‘blue’, what we mean is this… A blue object has atoms and molecules in its surface that vibrate in particular ways in response to the jiggling of the light

81. What is colour? When we say ‘That object is ‘yellow’, what we mean is this… A yellow object has atoms and molecules in its surface that vibrate in particular ways in response to jiggling of the light

82. Electromagnetic waves When particles with an electric charge oscillate, they create waves in the electric field, called electromagnetic waves Electromagnetic waves with different frequencies have different names: radio waves microwaves infra red light visible light ultra violet light X-rays gamma-rays

83. Light Light is an electromagnetic wave Visible light is generated by oscillations of electrons within atoms We learn about atomic structure by studying the light from atoms Each type of atom and molecule gives out a unique ‘spectral signature’ when ‘excited’. We can identify atoms by looking at the spectrum of emitted light

84. Electromagnetic waves Electricity Heat How it all fits together… Atoms

85. Homework?

86. Homework Activity If you are able to borrow one of the spectrometers try looking at : Different streetlights Clouds near the sun (look for dark bands in the spectrum) The lights around your house Light from your computer screen. Look at a white area, a red area, a blue area and a green area Look at a candle: then sprinkle some salt in the candle. Research: What is the coldest place on Earth?

87. One minute feedback On the back of your handouts! Rip off the last sheet Please write down what is in on your mind RIGHT NOW! A question? OK A comment? OK A surprising thought in your mind? I’d love to hear it!

88. On-line Resources www.protonsforbreakfast.org This PowerPoint ™ presentation. Handouts as a pdf file blog.protonsforbreakfast.org Links to other sites & resources Me going on about things

89. Next week will be much easier and there will be ice cream! Goodnight See you next week to discuss heat!

90. Breaktime Activity Use the spectrometers to look at the different sources of light Ask the helpers for help if you can’t see something like the spectrum below 700 nm 700 nanometres 0.7 thousandths of a millimetre 400 nm 400 nanometres 0.4 thousandths of a millimetre