Transcript light is a wave

Protons for Breakfast
Week 2 Light
November 2009
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!
This is the
most intellectually demanding week
• Different
Frequencies
– 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
Looking again at what we saw last week…
Lets take a look at some odd
phenomena…
• A balloon and a piece of
paper
Lets take a look at some odd
phenomena…
• 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
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
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?
• Radio 4 ‘long
Electric Charge
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)
Wavelength, Frequency
& Colour
Applet by DM Evans:
http://tre.ngfl.gov.uk/server.php?request=c2l0ZS5zZWFyY2g%3D&resourceId=13329&sf[userId][]=41910
Jelly Baby Wave
Machine
• Michael: don’t forget the 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 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
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?
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 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
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) 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…
Light from atoms
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 (7)
The Gherkinator
• The gherkin has a discrete spectral line at around 589 nm
• This indicates the presence of sodium atoms
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 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
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)
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 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…
Electricity
Electromagnetic
waves
Atoms
Heat
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
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
• 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