Transcript No Slide Title

Protons for Breakfast
Week 2 Light
November 2012
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?
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)
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
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
(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?
Thin wire
suspended in light
beam
Screen
LASER
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
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
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 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!
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
Goodnight
Next week will
be much easier
and there will be
ice cream!
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