protons for breakfast week 2 light november 2011

Protons for Breakfast

Week 2 Light

November 2011

In the event of an alarm sounding…

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?

Electromagnetic waves

Electricity

Heat

How it all fits together…

Atoms

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 repelIf particles have different signs of electric charge they attractThe 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

with electric charge

Particle

with electric charge

Interact by meansof an electric field

…but the steps happen very quickly

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

• What happens when you electrocute a gherkin?

The Gherkinator

Button of death

???????

What is light ?

A Question

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)

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

• 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

1 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022

Radio & TVInfra Red

Microwaves

Gamma-Rays

X-Rays

Ultra Violet

Frequency (Hertz)

1000 THz (Blue)400 THz (Red)

Electromagnetic spectrum

1 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022

Radio & TVInfra Red

Microwaves

Gamma-Rays

X-Rays

Ultra Violet

Non-ionising Radiation(generally not so bad) Ionising Radiation

(generally bad)

Frequency (Hertz)

Electromagnetic spectrum

• 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

Jelly Baby Wave Machine

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 experimentA 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?

LASERScreen

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

?

Our Experiment

What do we actually see?

LASER

Thin wire suspended in light

beam

Screen

This can only be explained if light is a wave

Interference

Overlapping Semicircles (1)

Wire

Screen

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

Interference Simulation

Interference simulation

Now with Red Light

What happens if we do the experiment with red light?

LASER

Thin wire suspended in light

beam

Screen

Diffraction Patterns

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

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

Afterbreak summary

• Light is a wave in the electric fieldFrequency

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 second186000 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

• 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’

• The light from the gherkin came from Sodium atoms

Light from atoms The Gherkinator

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

• 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)

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.

1 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022

Radio & TVInfra Red

Microwaves

Gamma-Rays

X-Rays

Ultra Violet

Frequency (Hertz)

Electromagnetic spectrum

ColdHot

3. Infra Red Light

H

Atoms & Molecules

H2

N

N

• A molecule is a collection of atoms stuck together electrically.

H

H

0

H20

H

N2

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

Some molecules vibrating

• Different types of molecular jiggling occur at different frequencies

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 wavesmicrowavesinfra red lightvisible lightultra violet lightX-raysgamma-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

Electromagnetic waves

Electricity

Heat

How it all fits together…

Atoms

Homework?

Homework

ActivityIf 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? OKA comment? OKA surprising thought in your mind? I’d love to hear it!

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

Next week will be much easier and there will be

ice cream!

Goodnight

See you next week to

discuss heat!

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