protons for breakfast week 2 light november 2011
TRANSCRIPT
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