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Protons for Breakfast Week 2 Light March 2010 Protons for Breakfast Week 2 Light March 2010

In the event of an alarm sounding… In the event of an alarm sounding…

Toilets… Toilets…

Parents and children… Parents and children…

Last Week’s talk • The scale and size of the Universe • Its very 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 This Week’s talk Light • Waves in the Electric field – I want you believe that light is a wave! This is Frequencies • Differentthe most intellectually demanding week – What is frequency? – What is resonance? • Relationship between light and atoms – All the light you see comes ‘fresh’ from an atom

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

Looking again at what we saw last week… Looking again at what we saw last week…

Odd phenomena… • A balloon and a piece of paper Odd phenomena… • A balloon and a piece of paper

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

Vd. G Vd. G

Vd. G Vd. G

The electrical nature of matter • Electric charge is a fundamental property of electrons 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 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

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

Now let’s move on… Now let’s move on…

Electric Gherkin Electric Gherkin

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

A Question What is light ? 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

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 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

A word about frequency (1) • 1 oscillation per second is called 1 hertz A word about frequency (1) • 1 oscillation per second is called 1 hertz

A word about frequency… oscillations per second is called a… 1000 (a thousand) (103) A word about frequency… oscillations per second is called a… 1000 (a thousand) (103) kilohertz (k. Hz) 1000000 (a million) (106) megahertz (MHz) 100000 (a billion) (109) gigahertz (GHz) 1000000 (a trillion) (1012) terahertz (THz) 100000000 (a million billion) (1015) petahertz (PHz)

Did you do your homework? • What was the frequency your favourite radio station? Did you do your homework? • What was the frequency your favourite radio station? • Radio 4 ‘long wave’ – 198 k. Hz • ‘Medium wave’ – 540 k. Hz to 1600 k. Hz • ‘FM’ stations – 88 MHz to 108 MHz • Digital Radio – 217 MHz to 230 MHz

Electromagnetic waves (1) • Electromagnetic waves can be generated with a vast range of 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

Electromagnetic spectrum Infra Red Radio & TV Ultra Violet Gamma. Rays Microwaves X-Rays 400 Electromagnetic spectrum Infra Red Radio & TV Ultra Violet Gamma. Rays Microwaves X-Rays 400 THz (Red) 1 102 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 Gamma. Rays Microwaves X-Rays Non-ionising Electromagnetic spectrum Infra Red Radio & TV Ultra Violet Gamma. Rays Microwaves X-Rays Non-ionising Radiation (generally not so bad) 1 102 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)

Wavelength, Frequency & Colour A wave has a Wavelength, a Frequency, an Amplitude, and Wavelength, Frequency & Colour A wave has a Wavelength, a Frequency, an Amplitude, and a Speed When it reaches our eyes we perceive COLOUR Applet by DM Evans: http: //tre. ngfl. gov. uk/server. php? request=c 2 l 0 ZS 5 z. ZWFy. Y 2 g%3 D&resource. Id=13329&sf[user. Id][]=41910

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

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

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

How can we prove that light is a wave? • Historically this ‘proof’ was 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 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 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 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 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 This can only be explained if light is a wave

Interference Interference

Semicircles (1) Wire Screen Semicircles (1) Wire Screen

Diffraction Patterns • The pattern seen on the screen depends on – The wavelength 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.

Overlapping crop circles Images Steve Alexander Copyright 2004 Overlapping crop circles Images Steve Alexander Copyright 2004

Interference Simulation Interference simulation Interference Simulation Interference simulation

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

Diffraction Patterns Diffraction Patterns

Light is a wave Wavelength is just less than one thousandth of a millimetre 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 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… Diffraction Grating • An array of fine lines…

Spectroscopic glasses • What do you see? Spectroscopic glasses • What do you see?

Break • Left-Hand Side – 15 minutes to look at some lights – 15 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 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 • Afterbreak summary • Light is a wave in the electric field – Frequency • 400 THz (Red) to 1000 THz (Blue) – Wavelength • 0. 7 thousandths of a mm (Red) to 0. 4 thousandths of a mm (Blue) – Speed • 300000 kilometres per second • 186000 miles per second

Afterbreak Questions • Why are some spectra made of discrete lines? • Why are Afterbreak Questions • Why are some spectra made of discrete lines? • Why are some spectra continuous? • What about light from molecules rather than atoms? • What makes an object coloured?

All light comes ‘fresh’ from atoms All light comes ‘fresh’ from atoms

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

Lets remind ourselves about atoms (1) • The internal structure of atoms Electrons • 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 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 simply ‘hitting’ an atom: How do we make light? • We make light by simply ‘hitting’ an atom: hard • Strike it with an other atom • Strike it with an electron • To make a wave at 1 petahertz we need: • Enormous forces • Very light particles • Enormous forces come the electric forces within an atom • Very light particles are electrons within an atom

Discrete Spectra Discrete Spectra

Light from atoms… If an atom or molecule is ‘unconstrained’ then • When it 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 (4) • We know about every type of atom that can Light from atoms (4) • We know about every type of atom that can exist. • And we know its spectrum…

Light from atoms Hydrogen Helium Lithium. We • know about every type of atom Light from atoms Hydrogen Helium Lithium. We • know about every type of atom that can exist. • Oxygen. And we know its spectrum… Carbon Nitrogen Neon Sodium Xenon

Light from atoms The Gherkinator • The light from the gherkin came from Sodium 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 (6) The Gherkinator • In my office…

Light from atoms (7) The Gherkinator • The gherkin has a discrete spectral line Light from atoms (7) The Gherkinator • The gherkin has a discrete spectral line at around 589 nm • This indicates the presence of sodium atoms

Continuous Spectra Continuous Spectra

Light from atoms in solids (1) • If an atom or molecule is ‘constrained’ 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 normal light bulb Light from atoms in solids (2) • The filament of a normal light bulb is heated to about 2500 celsius to make it give off ‘white’ light • When something is at about 800 celsius: its red hot • When its colder, it gives off only infra-red light. We can’t ‘see’ this light but we can detect it. • IR light is absorbed by molecules in our skin.

Electromagnetic spectrum Infra Red Radio & TV Ultra Violet Gamma. Rays Microwaves X-Rays Cold Electromagnetic spectrum Infra Red Radio & TV Ultra Violet Gamma. Rays Microwaves X-Rays Cold 1 102 Hot 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 Frequency (Hertz)

Infra Red Light Infra Red Light

Atoms & Molecules • A molecule is a collection of atoms stuck together electrically. Atoms & Molecules • A molecule is a collection of atoms stuck together electrically. H NN H 2 H N H H 20 0 2 H

What happens if you knock a molecule? • If a molecule is hit, the What happens if you knock a molecule? • If a molecule is hit, the atoms within a molecule vibrate. • Because atoms are thousands of times heavier than electrons they ‘ring’ with a much lower frequencies. • The light given off is in the infra red range of the spectrum. H 20

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

Colour Perception Colour Perception

What makes an object coloured? Yellow Blue What makes an object coloured? Yellow Blue

What is colour? (1) • When we say ‘That object is blue’, what we What is colour? (1) • 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? (1) • When we say ‘That object is yellow’, what we What is colour? (1) • 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

Summary (1) Electromagnetic waves • When particles with an electric charge oscillate, they create Summary (1) 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, visible & ultra violet light; X-rays and gamma-rays • The technology for generating and detecting these waves differs enormously

Summary (2) Light • Light is an electromagnetic wave • Visible light is generated Summary (2) 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 or 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 Electromagnetic waves Atoms Heat How it all fits together… Electricity Electromagnetic waves Atoms Heat

Homework? Homework?

Homework Activity#1: If you are able to borrow one of the spectrometers try looking Homework Activity#1: 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. Activity#2: • Try making your own spectroscope using an old CD and a cardboard ‘net’ Research: What is the coldest place on Earth?

One minute feedback • On the back of your handouts! • Rip off the 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!

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

Goodnight Next week will be much easier! See you next week to discuss heat! Goodnight Next week will be much easier! See you next week to discuss heat!

Breaktime Activity • Use the spectrometers to look at the different sources of light 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