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Transcript
Protons for Breakfast Light Week 2 November 2014 In the event of of… an attack of giant squids… Toilets… Parents and children… 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 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? How it all fits together… Electricity Atoms Electromagnetic waves Heat Looking again at what we saw last week… Odd phenomena… • A balloon and a piece of paper Lets take a look at some odd phenomena… • A balloon and an electroscope Van de Graaff Van de Graaff 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. 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 How do charged particles interact? It’s a three-step process… Particle Particle with electric charge Interact by means of an electric field …but the steps happen very quickly with electric charge How do we describe the world? The nature of interactions (1) Analogy with water level and water waves Now let’s move on… Electric Gherkin The Gherkinator • What happens when you electrocute a gherkin? ??????? Button of death A Question What is light ? Lets take a look at some odd phenomena… • A balloon and an electroscope Lets take a look at some odd phenomena… • A balloon and an electroscope • Wiggling the balloon… • Causes the electroscope to wiggle 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 Frequency Frequency • 1 oscillation per second is called 1 hertz Frequency… oscillations per second is called a… 1000 (a thousand) (103) kilohertz (kHz) 1000000 (a million) (106) megahertz (MHz) 1000000000 (a billion) (109) gigahertz (GHz) 1000000000000 (a trillion) (1012) terahertz (THz) 1000000000000000 (a million billion) (1015) petahertz (PHz) How does Radio work? • Wire telegraph is a kind of a very, very long cat. • You pull his tail in New York and his head is meowing in Los Angeles. • Do you understand this? • And radio operates exactly the same way: Albert Einstein • You send signals here, they receive them there. • The only difference is that there is no cat. Did you do your homework? • What was the frequency your favourite radio station? Electric Charge • 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 Electromagnetic waves (1) • Electromagnetic waves can be generated with a vast range of frequencies, but in a vacuum, they all travel at the same speed • The complete range is called the electromagnetic spectrum • Different frequencies of electromagnetic waves have different names • Different frequencies require different types of equipment • to generate • to detect Electromagnetic spectrum Infra Red Radio & TV Ultra Violet GammaRays Microwaves X-Rays 400 THz (Red) 1 101 102 1000 THz (Blue) 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 Frequency (Hertz) Electromagnetic spectrum Infra Red Radio & TV Ultra Violet GammaRays Microwaves X-Rays Non-ionising Radiation (generally not so bad) 1 101 102 Ionising Radiation (generally bad) 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 Frequency (Hertz) Jelly Baby Wave Machine • Michael: don’t forget the Jelly Baby Wave Machine! Jelly Baby Wave Machine • the wave moves from one place to another, • the jelly babies just move up and down Is light really a wave in the electric field? 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. Young’s experiment A double slit • This is how Young conceived of the experiment Our Experiment • • A laser gives light with just a single frequency What would we expect to see if we shine it at a screen? Screen LASER 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? ? Screen Thin wire suspended in light beam LASER Our Experiment What do we actually see? Thin wire suspended in light beam Screen LASER This can only be explained if light is a wave Interference Wire Screen Now with Red Light What happens if we do the experiment with red light? Thin wire suspended in light beam Screen LASER Wire Screen Diffraction Patterns The pattern seen on the screen depends on The wavelength of the light The diameter of the wire . Seeing these bright and dark bands establishes that light has a wave nature. Interference Simulation Interference simulation Light is a wave Wavelength is just less than one thousandth of a millimetre 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 Diffraction Grating • An array of fine lines… Spectroscopic glasses • What do you see? 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 What I hope you saw! • Filament Lamp • Fluorescent Lamp •700 nm •700 nanometres •0.7 thousandths of a millimetre •400 nm •400 nanometres •0.4 thousandths of a millimetre Photo credit http://home.comcast.net/~mcculloch-brown/astro/spectrostar.html 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 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? All light comes ‘fresh’ from atoms 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! 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 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 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 (1015 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 1. Discrete Spectra 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. Light from atoms • We know about every type of atom that can exist. • And we know its spectrum… Light from atoms ‘Atomic Fingerprints’ Hydrogen Helium Lithium • We know about every type of atom that can exist. • And we know its spectrum… Oxygen Carbon Nitrogen Neon Sodium Xenon Light from atoms The Gherkinator • The light from the gherkin came from Sodium atoms Button of death Light from atoms (6) The Gherkinator • In my office… Light from atoms The Gherkinator • The gherkin has a discrete spectral line at around 589 nm • This indicates the presence of sodium atoms 2. Continuous Spectra Light from atoms in solids (1) • 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 (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. Electromagnetic spectrum Infra Red Radio & TV Ultra Violet GammaRays Microwaves X-Rays Cold 1 101 102 Hot 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 Frequency (Hertz) 3. Infra Red Light Atoms & Molecules • A molecule is a collection of atoms stuck together electrically. H NN H2 H N H H 20 0 2 H 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. H20 Vibrations of CO2 and H20 • Different types of molecular jiggling occur at different frequencies Water H20 Carbon dioxide C02 Colour Perception What makes an object coloured? Yellow Blue 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 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 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 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 How it all fits together… Electricity Atoms Electromagnetic waves Heat Homework? 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? 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! Goodnight • blog.protonsforbreakfast.org This PowerPoint ™ presentation. Questions and Answers as a pdf file Links to other sites & resources Me going on about things See you next week to discuss heat! Next week will be much easier and there will be ice cream!