Protons for Breakfast Week 2 Light November 2009

Protons for Breakfast

Week 2 Light

November 2009

Restricted Commercial

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!

• Different Frequencies– What is frequency?– What is resonance?

• Relationship between light and atoms– All the light you see comes ‘fresh’ from an atom

This is the most intellectually demanding week

How it all fits together…

Electromagnetic waves

Atoms

Heat

Electricity

Looking again at what we saw last week…


Lets take a look at some odd phenomena…

• A balloon and a piece of paper



• A balloon and an electroscope

VdG

VdG


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


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



• A balloon and an electroscope



• A balloon and an electroscope• Wiggling the balloon…• Causes the electroscope to wiggle



• The balloon is a source of electric waves (technically electromagnetic) waves.

• The waving electroscope is a detector of electric waves

Frequency

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)

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)


Wavelength, Frequency & Colour

Applet by DM Evans: http://tre.ngfl.gov.uk/server.php?request=c2l0ZS5zZWFyY2g%3D&resourceId=13329&sf[userId][]=41910

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

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


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

http://www.ngsir.netfirms.com/englishhtm/Interference.htm



Now with Red Light

What happens if we do the experiment with red light?

LASER


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 can separate light into its different frequencies and 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


• Fluorescent Lamp

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 some spectra continuous?

• What about light from molecules rather than atoms?

• What makes an object coloured?

All light comes ‘fresh’ from atoms

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• ‘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 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

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 exist.• And we know its spectrum…

http://www.colorado.edu/physics/2000/applets/a2.html

http://www.colorado.edu/physics/2000/applets/a2.html

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

• 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 (7)The Gherkinator

• The gherkin has a discrete spectral line at around 589 nm• This indicates the presence of sodium atoms

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 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.

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)


ColdHot

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 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.

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? (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 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

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• 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…

Electromagnetic waves

Atoms

Heat

Electricity

Homework?

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 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 PowerPoint ™ presentation.–Handouts as a pdf file

• blog.protonsforbreakfast.org –Me going on about things

• links.protonsforbreakfast.org –Links to other sites & resources

http://www.protonsforbreakfast.org/

http://www.protonsforbreakfast.org/

http://blog.protonsforbreakfast.org/

http://blog.protonsforbreakfast.org/

http://links.protonsforbreakfast.org/

http://links.protonsforbreakfast.org/

Next week will be much easier!

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



Documents

Protons for Breakfast Week 2 Light November 2009