PROTONS FOR
BREAKFAST
AUTUMN 2013
WEEK 3
HEAT
1
This is what you said on the feedback forms in Week 2
You said…
Michael said…
Great presentation again –
You are welcome. Thinking about your comment – it
thank you. Somehow it
seems to apply to the scientific description of the world
seems so logical yet so
in general!
complicated.
Absolutely amazing – I’m so glad I
I am glad you are enjoying it: I hope you can
brought my son to the lectures.
tell that I enjoy it too. Physics is indeed
Physics is so exciting!!!
exciting.
What do you think is
I really don’t have a clue! My guess is that it will be
going to be the next
something biological, but I would love it to be something to
scientific breakthrough? help us make energy without polluting the atmosphere.
 Thanks, another  You are welcome
great week.
 Yes: well observed. I think you will have seen something like
 Why, when
this:
looking through
the special
specs, were
there 2
With blue nearest to the middle and red the furthest away. The
‘rainbows’? I
basic reason is that the spectra are created by inference
tried closing one between light which goes one way around the lines on the
eye and then the glasses and light which goes the other way around the lines. For
other, which
example,
made no
 the bright red lights on the far left and far right are
difference –
created by the constructive interference (adding up) of light
there were still
with a wavelength around 0.0007 millimetres.
2 mirror image
 the bright blue lights towards the middle are created by the
‘rainbows’
constructive interference (adding up) of light with a
(spectra?) either
wavelength around 0.0004 millimetres.
side of the light By the way the glasses are available from here:
http://www.rainbowsymphonystore.com/digrglli500l.html#
source…..
You talk about
It is important to understand that ’colour’ is a human sensation i.e.
the colour
it is something we experience. So I think your question can be
spectrum but
translated as:
how do we see
 What spectrum of frequencies of light elicits the experience we
white light and
call ‘white’?
white objects?
‘White’ is what we experience when the three types of cone in our
eyes are roughly equally stimulated. There are many ways this can
be achieved e.g. with three specific frequencies of light or with a
wide range of frequencies.
2
You said…
 What is the
coldest place
on Earth?
 How old are
you?
 What is an
atom?






Michael said…
 That’s the homework! I can’t tell you now! And anyway, you didn’t
clarify if that was the coldest naturally-occurring place on
Earth, or the coldest man-made experiment?
 I am 53.875 years old.
 An atom is a tiny particle from which all the substances on
Earth are composed. There are about 100 different types of
atom.
Thank you!
 You are welcome
I had never understood
 They are very unspectacular – just a tower with a wire.
about radio and TV
Read about the Radio 4 transmitter here:
waves – please could we
see a photo of one of
the towers for radio
wave?
How is a spectrum
‘owned’ by certain
bodies, who has the
right to sell it and why?
Will that be the same in
space – once we can
travel to planets and
inhabit them will we
have a new imperial age?
(Lady Gaga to have her
own planet for
concerts?!) courtesy of
Virgin Galactica.
- I loved the powers of
ten video and have had
it at breakfast several
times.
Yes please a
spectrometer.
http://en.wikipedia.org/wiki/Droitwich_Transmitting_Station




For mobile phone frequencies, the companies pay the
government money to enforce laws that stop anyone else
using those particular frequencies. This allows (for
example) Vodafone to run a mobile phone business
without interference from another company, or other
pieces of technology. We will have an expert from
Vodafone here in Week 5 and you can ask him.
It is a nice idea, but in fact human beings are tied
symbiotically to the surface of this one planet. It is
highly unlikely that humans can exist for extended
periods i.e. centuries - away from the surface of the
Earth, at least not without supplies of energy or food
from the Earth.

I hope you borrowed one. The odd asymmetric shape
means that you look at light on only one side of the
bright central light that you saw with the diffraction
glasses.
3
You said…
 I love it.
 Where do you
find the
songs
 Can someone
make a new
colour.
 This is so
useful for
physics
revision.
Thank you.




Michael said…
Great. 
Details of the songs are here:
http://protonsforbreakfast.wordpress.com/songs/
This page has details of the artists involved and contains links to
the iTunes store.
Mmmm. What a profound question. Your visual system can
present you with a bewilderingly large number of colour
sensations. We already have names for the general types of
sensation: red, green, light blue, dark orange, pinky-purple etc.
Is there an un-used colour sensation that could be named and
‘discovered’? I guess it’s possible!
I am glad – and relieved - it is useful as well as enjoyable 
4
You said…
 1022 tops out
but what of
BLUE
SHIFTS.
 1 Hertz = 1
wave/sec.
What’s below 1
Hertz. Do you
get 1/10th,
1/100th etc. of
a Hertz? Do
these waves
have
names/uses?
One wave a
week!
 What’s the
slowest wave
we can
currently
detect?
 I think the
learning value
of the wave
experiment is
worth impaling
the jelly
babies!
Michael said…
This article has a review of ‘cosmic rays’:
http://protonsforbreakfast.wordpress.com/2011/05/16/cosmicadventure/
 And this article describes the latest results from the alpha magnetic
spectrometer:
http://protonsforbreakfast.wordpress.com/2013/04/07/cosmic-rayssurprise-us-again/
 As I think I mentioned, it gets harder and harder to make high
frequency waves because a charged particle has to be accelerated at
an ever greater rate which requires bigger and bigger forces.
To make a wave with a frequency of 1015 hertz the forces on
electrons on the outer parts of an atom are large enough. To emit Xrays requires larger forces which are experienced by the inner
electron shells of heavy atoms. Gamma rays (of which more in Week
6) come from the nucleus itself. Here protons (which are ~2000
times heavier than electrons) must be accelerated by the combined
effects of electricity and the immensely powerful strong force. And
we just don’t know how to get stronger forces than this!
 Blue shifts occur when the object which emits the radiation is
heading towards us when it emits the light. This could affect the
frequency in an extreme case by perhaps a factor of two or three –
but I don’t think it could increase the frequency by a whole factor of
10 – but I could be wrong!
 Red shifts occur when the object which emits the radiation is
heading away from us when it emits the light. As we look out into the
Universe almost everything seems seem st obe receding from us. Red
shifts of visible light by factors up to about 8 have been observed
 Yes Extra Low Frequency (ELF) and Ultra Low Frequency (ULF)
waves do exist and can be measured. However these waves are
everywhere all the time and so they are difficult to detect against
the ever-changing background electric fields. Additionally if you
wanted to encode the wave to send a message by (say) changing the
frequency, it would take a very long time!
 By ‘slowest’ I think you mean lowest frequency. All electromagnetic
waves travel in a vacuum with the same speed – the speed of light! I
think we can detect very very low frequencies, certainly milli-hertz,
but I think there are not many applications where this is considered
useful.
 I am glad you considered their sacrifice worthwhile: because there
will be further impaling in Week 5.

I like watching the Gherkinator.
 Enjoyed talk, it reinforced the work
we did on the same topic in school 3
weeks ago. Much clearer now.
5
And I enjoy firing it up!
 Wow – this course might be relevant to
school work!
You said…
 Why do LED
lights use
less energy
than
traditional
filament or
fluorescent
ones?
Michael said…
It is difficult to describe exactly, but roughly:
 Traditional incandescent bulbs electrically heat a fine tungsten
filament to get it ‘hot’. We then rely on random high-frequency
electronic jiggling to produce light. For normal bulbs about 2% of
the electrical energy emerges as light. For Tungsten-Halogen
bulbs (in which the filament can be heated to a higher
temperature) this can reach 4%. The rest of the energy is
emitted as heat – which is OK if that energy then heats your
home or office – but not OK if you then need to cool your office
more!
 The challenge is to engineer the oscillations that emit light in a
way that is less chaotic – so that a bigger fraction of the incident
energy emerges as light.
 Inside a fluorescent tube is mercury vapour. First a ‘spark’ is
struck in the gas and when the atoms of mercury are struck by
electrons they vibrate and emit characteristic frequencies of
light: these were the striking intense bands of colour you could
see with the ‘diffraction grating glasses’. Also the atoms emit
ultraviolet light which is absorbed in a coating on the inside of the
tube. This then fluoresces (hence the name) and emits visible light
with a wide range of frequencies.
 Inside an LED light is an LED (obviously!) which stands for lightemitting diode. This is a device which consists of a junction
between two types of semiconductor – in the junction region
electronic charges can be induced to oscillate in a highly efficient
manner with approximately a single frequency. Changing the
semiconductor material changes the emission from infra-red, to
red, to green, to blue, to ultra violet. To make a white light one
can either have:
o three colours of LED – red, green, and blue – or
o more commonly we coat a blue or ultraviolet LED with a
phosphor which emits at a wide range of frequencies just like in
a fluorescent light.
This is absolutely rubbish! JUST KIDDING! Lots The idea that you should realise you
of fun, really entertaining and helped me realise
know more than you thought you did
I know a lot more than I thought I did. Thanks!
is heartwarming. 
GCSEs here I come!
6
You said…
 Very good.
Enjoyed it a
lot. Learning
lots from
this!
 What
medium
does light
need to
travel
through?
Michael said…
 Great
 Light (obviously) needs a ‘transparent’ medium. But what does
‘transparent’ mean?
 Vacuum Well light (and radio waves, X-rays etc.) can travel
through a vacuum i.e. a region with no atoms. So this tells us
something about the nature of ‘empty’ space.
 Gases Visible light can travel reasonably well through most gases
but ultraviolet light sets the outer electrons vibrating too
strongly and is absorbed and then re-mitted – a process called
scattering. Infrared light is absorbed and the scattered by the
motion of whole atoms within molecules.
 Solids Most solids thicker than about 0.001 mmm are opaque to
visible light, but surprisingly light can actually travel through some
solid materials. These ‘transparent’ materials are all electrical
insulators and are typically crystals, although normal glass is not
crystalline. In solids there is a high density of electric charge, but
in some specific arrangements of atoms and molecules, the
vibrations of the electrons in the outer orbits of atoms are not
converted into atomic vibrations i.e. heat, but instead re-radiate
the wave ‘intact’. All the wave proceeds in tact it is considerably
slowed – light travels through glass with only about two thirds the
speed with which it travels in vacuum.
Thank you, very interesting? I learnt a lot. 
 What is the distance
 We call the nearest star Proxima Centurae and it
between earth and the
is measured to be about 4.23 light years away.
nearest star?
Travelling at the speed of our fastest man-made
 How far can a radio wave
object (about 140,000 km per hour or 39 km per
travel?
second!) it would take more than 32,000 years to
 Thoroughly enjoyed this
reach the star.
week. Very informative. Will
http://en.wikipedia.org/wiki/List_of_nearest_stars
definitely help with my
 We don’t know of any limit to how far a radio wave
physics school work. Keep up
can travel – we have detected radio waves that
the good experiments being
were emitted 300,000 years after the big bank –
fun to interact as well.
i.e. more than 13 billion years ago.
 Thanks 
7
You said…
Michael said…
 Can you include Einstein said
this Einstein’s
“You see, wire telegraph is a kind of a very, very long cat. You
explanation of
pull his tail in New York and his head is meowing in Los Angeles.
‘How radio
Do you understand this? And radio operates exactly the same
wave works’.
way: You send signals here, they receive them there. The only
Please. Thanks.
difference is that there is no cat.”
 Is the light
 Is sound from a football crowd the same as sound from a band
from a fire
of flute players? Well yes, it is still sound but has quite a
the same light
different character. The sound from a football crowd consists
from a laser?
of lots of individual noises which add chaotically together. In a
 Thank you, I
flute band, the individual tones are pure notes and each flute
found this
plays the same sound.
lecture on
 Similarly, light from a flame and light from a laser are both
light
waves in the electric field i.e. light. The light from a flame is
fascinating!
chaotic in that the emissions from each atom are independent
of each other. In a laser, atoms emit light in synchronisation
with each other yielding a ‘pure tone’ i.e. a single frequency of
light.
 You are welcome 
Is colour blind a
Please see the explanation below. I hesitate to use the word
fault in perception
‘fault’ because I know several friends who have different colour
or receptors in the
vision in each eye! In some people it is a ‘problem’ of which they
eye?
would have no consciousness without being ‘diagnosed’ by a
doctor.
 What part of
 Radio telescopes are ’listening’ to radio emissions from stars
the
and galaxies in the same way that visible light telescopes are
electromagnetic
‘seeing’ visible light emissions. In general visible light only
spectrum are
comes from hot objects but radio telescopes can see cooler
radio telescopes
objects. Also radio waves can travel through some kinds of
investigating and
dust clouds and allow us to see objects that are hidden to
what sort of
visible light telescopes.
information are
 The telescopes listen at a wide range of frequencies from a
they gathering?
few MHz up hundreds of GHz in the microwave part of the
spectrum.
http://www.jb.man.ac.uk/research/
 You can read through this web site about Jodrell Bank Radio
Telescope in Cheshire. To me the site is spectacularly
uninformative, but it has nice pictures.
8
You said…
Michael said…
 Based on the
 Yes. In normal colour vision, light stimulates three different
theory, colour
types of so-called ‘cone’ cells in the retina of our eyes. We
blind is just a
generally call these cells ‘red’ ‘green’ and ‘blue’ but in fact they
mild incorrect
all respond to a wide range of wavelengths of light. Our
interpretation
sensation of colour depends on the relative stimulation from
of colour.
these cells. The graph below shows ‘sensitivity of the three
 Is there the
types of cone cells to light of different frequency.
colour
spectrum for
various
organic
matter?
 When they say
‘someone has
chemistry
with someone’
does that
mean they
If we ignore the blue cells for the moment then:
have actually
o If the red cells are stimulated more than the green cells then
electrical
we have a sensation we call ‘Red’
charge is
o If the red cells are stimulated about the same as the green
magnified in
cells then we have a sensation we call ‘Yellow’
the body?
o If the red cells are stimulated less than the green cells then
we have a sensation we call ‘Green’
 What the graph above shows is that the red and green cells
have a very similar response. In people who are colour blind, the
response curves may be more similar giving them a weaker
ability to distinguish between shades of red and green and
yellow. In some people one type of either red or green cell is
completely absent and these people cannot distinguish reds and
greens at all.
 I am not sure that I understand you comment about organic
matter. In general visible light is emitted from electrons around
individual atoms and is characteristic of that atom. Organic
matter means molecules built around carbon atoms and
commonly including hydrogen, oxygen and nitrogen atoms. The
way these compounds absorb and emit infrared light depends on
the details of how the molecules are bonded together.
 When we say a ‘couple have chemistry’ I suspect we are saying
that in same way that chemistry at school was mysterious but
seemed to work. So we have no clue as to what they see in each
other, but they seem to get on.
9
You said…
Michael said…
 You were right when you said  Sorry: Things should be easier tonight.
at the beginning of tonight’s
 Don’t worry about forgetting things.
talk “TONIGHT WILL BE
 The first stage of learning is just ‘becoming
DIFFICULT…..”.
familiar’. And in the same way that I forget the
 So difficult for me to
names of people I was introduced to 1 minute ago
understand – let alone
– so it’s easy to forget labels for new ideas.
remember – that I can’t even  But just thinking about these things develops
formulate a question from
familiarity, and so at some point in the future
the mass of information.
you may meet these ideas again, and feel
 NOT to work tho’ cos it was
differently: it might feel like meeting an old
interesting (but sadly
friend 
forgettable) all the same.
 Great
 When you ‘look at a light’ you have an experience that you call
presentation.
‘seeing the light’. It involves:
If you look at
o Electromagnetic waves from the ‘light-emitting object’
a light, what
travelling through the air to your eye.
do you see.
o The focussing of the waves onto your retina to create an image
 And do you
of the object.
watch Big
o The stimulation of different types of cells each of which
Bang Theory.
responds differently depending on the range of frequencies
present in the light.
o The neural processing of these data to elicit your sensation
o So what you ‘see’ is the result of complex process involving
atomic physics, optics, bio-chemistry and neuroscience. WOW!
 I wouldn’t say that I watch it, but my children do and I have
seen some episodes.
 So, if we have a set number of
 No more than the Earth could run out
electrons and channel electricity not
of water if we all ran a bath at the
‘create’ it, could we theoretically run
same time. We could run out locally –
out? (If it was all being channelled at
but not on the scale of the whole Earth.
the same time).
 No excuse required, I love geographers

 Please excuse me if this is a ridiculous

question. I am a geographer.
Lots of amazing facts and coherent
explanations. Elevating. Amazing!
10

You are welcome 
You said…
Michael said…
 Really interesting –
 Yes, the connection between light and atoms is very
learnt so much this
profound. I only learned it – or rather it only ‘clicked’ –
week. Changed how I
in my second year at University and then I just
see and think about
thought ‘WoW!’
light. It is a beautiful
 Wiki (of course) has all the details, but basically blue
concept.
light in frequency range 670 THz to 714 THz
 How is light used as a
(wavelength range 420-448 nm) can break down the
therapy (as a nurse).
bilirubin, a chemical which accumulates in the blood
I’ve seen how
about half of all babies because the enzyme which
ultraviolet light is used
normally breaks it down has been underactive during
to treat jaundice in
pregnancy. Wiki tells me that :
babies? – What does
“Phototherapy works through a process of isomerization
the UV light do?
that changes trans-bilirubin into the water-soluble cisbilirubin isomer.”
 In honesty, I am not sure what that means.
 Light does not
 Mmmm.
move, as the
 The whole point of the giant Jelly Baby Wave Machine was to
jelly bean
show that while each kebab stick just twists up and down, a
experiment
wave-like disturbance travels without transporting any Jelly
proved, it is the
Babies along the wave direction
force on the
 Similarly, the electric field in each location only goes up and
adjacent atom
down i.e. increase and decreases, but the wave – which we call
that is visible as
light or radio waves or X-rays depending on its frequency –
light.
travels through space.
 However, why
 Amazingly the filament of an incandescent light heats up to
does it take
about 2500 °C in just a small fraction of a second. In a
longer for light
compact fluorescent bulb, an electric charge needs to build
from compact
up to generate an initial spark through the tube. That is
fluorescent light
cause of the delay of a second or so before they switch emit
to get to us than
light.
old
incandescence.
11
You said…
Are the spectral
lines from
distant planets
red shifted?
And if so, how
do we establish
which element
has emitted
them?


Michael said…
No, the relative motion of planets in our solar system is not fast
enough to cause significant re-shift. But the identification of an
element doesn’t rest on a single emission line, but rather on the
pattern – akin to the way the details of a fingerprint identifies a
person. So if one emission line occurs in particular relationship with
another one, then we can identify the atom even if both are shifted
from where they would be detected on Earth. The picture below
from Wikipedia gives you the idea
http://en.wikipedia.org/wiki/Redshift
How far can light 
travel before it
red shifts to
infrared?
Do gamma ray

photons get
absorbed by
glass? (Three

drawings on
page).

The frequency of light does not change with distance
travelled. If red light with a frequency of 400 THz is
emitted by a star then we will also see red light with a
frequency of 400 THz.
However if the star that emits the light is moving away from
us very fast then light wave is ‘stretched’ and appears to
have a lower frequency – the so-called ‘Red Shift’
Wikipedia tells me that distant stars appear to be moving
away from Earth at speed of 21 km/s per million light years
distant and since some stars can be billions of light years
away, they appear to recede at amazingly high speeds
Gamma rays are only weekly absorbed by glass – we will try
this experiment in Week 6. If I skip it – please remind me.
12
You said…
 I’m studying
‘A’ level
physics – still
don’t really
understand
how light can
be both a
wave and a
particle?
 What’s wrong
with people
that are
colour blind?
 Why is mixing
paint colours
different to
mixing light
colours?
Michael said…
Well don’t worry about it, you are in good company. The following may or
may not help:
 When people say that light can be a ‘particle’ they do not mean a
particle as you or I would use the word. It is a technical physics
description. You and I would not use the word ‘particle’ to describe an
object which had no mass, and which was created by the billion every
second and destroyed nanoseconds later.
 All light is emitted by atoms and travels as a wave – there is no doubt
about this. But when light reaches an object we find even though the
wave is extended in space – potentially over vast distances – the light
is always absorbed by a single atom in one location – as if it were hit
by ‘a particle’. We can describe this process – where the wave is
extended over space but is then detected in only a single place - but it
doesn’t have a simple analogy in our normal experience.
 I have mentioned about colour-blindness somewhere up above.
 Let me give you an example of light mixing. You experience the
sensation of ‘red’ when light with a frequency of roughly 400 THz
enters your eyes.
o If it enters your eyes directly from an emitter – such a ’red’ LED,
then the process is pretty straight-forward stimulation of the ‘cone’
cells.
o However if you look at an object which is illuminated by white light
and experience the sensation of ‘red’, then the to create the red
sensation, the surface chemicals (pigments and inks) must have
subtracted (i.e. absorbed) the blue and green light present in the
white light in the room.
 So looking at emitting objects – the primary colours are Red Green
and Blue which stimulate directly our three types of cone cells,
 Looking at reflected light the primary colours are :
o Cyan (which subtracts Red light from white light) leaving
components which stimulate mainly the ‘blue’ and ‘green’ cone cells.
o Magenta (which subtracts Green light from white light) leaving
components which stimulate mainly the ‘blue’ and ‘red’ cone cells.
o Yellow (which subtracts Blue light from white light) leaving
components which stimulate mainly the ‘red’ and ‘green’ cone cells.
So to get ‘red’ one uses an ink containing magenta (which removes green)
and yellow (which removes blue) leaving just the red light
Sorry it’s so complicated!
13
You said…
Michael said…
 How long does it take to

answer all of these

questions?
 How do we know that the
colours we see are the same?
As in we call trees green but
to some people green could
look red but they wouldn’t
know because all their life

they would have called it
green.
 What is your favourite colour
and why?






Several hours.
This is a profound philosophical question which
has its roots in physics, but is really about the
foundations of language and the ability to know
what someone means when they refer to a
‘shared’ experience. I seem to recall that
Wittgenstein – amongst others – wrote
extensively on this issue in his later years.
In the case of colour, I have friends whose
experience of ‘red’ is not the same in each eye,
so we can guess that between people, variations
exist. However, I don’t think the variability can
be too great otherwise we would eventually
realise that something wasn’t quite right.
 My favourite colour is red, but I don’t know why.
Can you “light up”  Just pickled ones: it’s the massive amounts of salt (NaCl)
any vegetables,
that make gherkins taste so good and which gives us the
or only pickled
yellow glow.
ones?
 I met some chemist from Imperial College who washed the
salt out of the gherkins by soaking them in water for weeks
and then replaced the sodium chloride (NaCl) with potassium
chloride (KCl) to get a purplish glow.
Really excited to be inside this
 You are very welcome. Your taxes (well not
handsome new-ish building, and I
just yours – other people helped too!) have
realised this evening that I feel
paid for the building.
as if I am at one of the Royal
 I did once apply to do the Christmas Lectures
Christmas lectures – which I
at the RI but they said I wasn’t an academic
have only seen on TV. Trying
and so they wouldn’t look at the application.
hard to take it all in – many
Hey Ho.
thanks.
Last week you suggested that we should
 Mmmmm. Fair point.
abandon the myth of the ‘mad scientist’.
 I am not sure what you mean. Did you
However, after the jelly baby
mean that the fact that we have
experiment I felt a bit of eccentricity
evolved to have a visual system
helps put across a concept.
designed for the spectrum of light
Q: Where does the sun come into the
emitted by our nearby star?
equation?
Loved the jelly baby
 You welcome: It will be back in Week 5.
example – great visual  When I am talking – which I can do endlessly! – it can be
to help understanding.
hard to think. It is only when I stop talking that people
Thanks!
get a chance to relax and ‘think’.
Thanks for making a very complicated subject fun.
 You are welcome.
14
You said…
Gherkin. If
the current
is AC, why
does only
one end
light up?
Michael said…
To be completely honest I don’t know why the Gherkin only lights up at
one end. Although I am curious – it is one of those questions that has
never made it up my priority list to answer. But I have thought about it.
Here are some observations:
One: The end which lights up changes from one gherkin to the next.
Two: We are using AC current which travels both ways through the
Gherkin, reversing every 1/100th of a second.
Three: You can see this more clearly at lower voltage – is that it takes a
few seconds for the light to emerge.
Putting these things together I think what happens is that:
 Initially the salty water makes a good electrical contact with the
tines of the fork.
 As the gherkin heats up the water evaporates and at some point
direct electrical contact gets very poor and a spark (a region of hot,
ionised gas since you ask) jumps from the tine to the gherkin – and it
is this which emits the light.
 The gas in the spark contains sodium atoms and so gives out yellow
light which then glows eerily inside the gherkin.
 Once this happens at one end, the current is reduced and it becomes
unlikely to happen at the other end as well. After a few seconds it
stops because the electrical connection becomes too poor.
And that is my guess 
15
You said…
Michael said…
 Q1: Re diffraction. If it’s  Great question, and yes it matters. One common way
due to the response of
to prepare these slits is to print them on to a
the atoms on the edges
transparent slide.
of the wire/slit, does the
o So if you print them in black, then that makes a
material the wire/slit is
standard slit. By ‘black’ we mean an ink containing
made of have an effect
atoms and molecules that absorb light of all
on the diffraction
colours.
observed?
o But if you created the slit with ink that didn’t
 Q2: The spectroscopy, if
absorb light – then it would be transparent – and
you are looking at light
you wouldn’t have a slit!
emitted by something
o (Re-reading this I am not sure how clear I have
composed of several
been, but I think it just about makes sense)
elements (e.g. the
 Yes it does get really confusing – like having two
atmosphere of a planet)
fingerprints present at the scene of a crime. But as
how can you interpret the
my answers to a couple of questions earlier indicate,
spectrum? As they
it is possible to unravel complex mixed spectra by a
overlap, doesn’t that get
process of guessing and elimination. Once you find
really confusing?
one element which ‘fits’ (there are only about 100
 Comment: Thanks very
possibilities and a much smaller number of likely
much for the glasses! I
candidates) then the resulting spectrum gets
will pass on to my son’s yr
simpler, and it becomes easier to identify the
1 class who are doing a
remaining elements.
topic on “light and dark”
at the mo.
 So ghosts are made of ultraviolet  Interesting idea except that the light would
light to gamma rays since we
be quickly gone – and if it went through our
can’t see them and they can pass
eyes we wouldn’t see it! The questions is this:
through us!
what would be emitting the UV light and
 What about the glasses with
gamma rays?
gratings, didn’t the light
 The region that you saw as red on the left
‘increase’, in your experiment,
and right of the central bright patch was
then why doesn’t the light coming
where the light with frequency 400 THz was
through the glasses ‘increase’?
re-inforced. Light with frequency of 700
THz was cancelled in these regions leading to
you seeing no ‘blue’ light and so you saw only
‘red’ light.
I found it
 Thank you
electrifying.
16
You said…
Michael said…
 If Na2CO3
 Sea salt is not made by an acid reaction with sodium chloride.
(sodium
Actually I have never quite understood this, so consulting
carbonate)
Wikipedia I read that currently ‘we’ think the sodium arrives
combines with
in the sea by being washed in from the rivers of the world,
HCl hydrochloric
and the chlorine derives from dissolved volcanic gases.
acid to make
 The evaporation of ocean water and the perpetual ‘washing’ of
NaCl + CO2 +
rocks resulted in an increased concentration of salt over
H2O, then how is
geological time periods, but the salinity is now reasonably
sea salt made in
constant.
the absence of
 Both co-valent and ionic bonding are basically electrostatic in
acid, and since
origin.
H2O (water) is
o In covalent bonding – which I think I described last week –
neutral pH.
the electron orbit is shared between two atoms resulting in
 What is the
electric charge in between the two atoms which pulls the
difference
nuclei inwards towards each other.
between ionic
o In ionic bonding the electric field of one atom completely
and covalent
strips the electron from the other atom resulting in a
bonding?
positive ion and a negative ion which are then electrically
 Why are atom
attracted to each other.
shells 2 then 8?  Most covalent bonds are not purely covalent: pure covalent
And can you
bonds only occur between identical atoms (H2, N2 etc.). In cowrite Ra(86) as
valent bonds between different types of atoms electrons
atomic
prefer orbits which draw them closer to one atom or the
structure?
other. So normal co-valent bonds are slightly ionic in
character.
The massive nucleus of a radon atom contains 86 protons and so
a radon atom has to accommodate 86 electron orbits.
The electronic structure is described by a diagram such as that
above, or as ([Xe] 4f14 5d10 6s2 6p6) i.e. a core equivalent to a
Xenon atom
o + 14 electrons in orbitals called 4f
o + 10 electrons in orbitals called 5d
o + 2 electrons in orbitals called 6s
o + 8 electrons in orbitals called 6p
The numbers and letters describe the detailed shape of the
orbits.
http://en.wikipedia.org/wiki/Radon
17
You said…
Do light waves

only oscillate in
a vertical plane
or do they go in
a horizontal one
as well.
Michael said…
Great Question , and yes they do vibrate in both ways
If the electric charge that generates the light wave moves
up and down, then it generates a wave in the electric field
that first points upwards and then points downwards.
 If the electric charge that generates the light wave moves
left and right, then it generates a wave in the electric
field that first points left and then points right.
 The two types of wave are called the polarisations of light.
Normal light is un-polarised i.e. a mixture of up-and-down and
left-and-right polarisations.
 We exploit the polarisation of light in many ways.
o Sunlight reflected from shiny surfaces is slightly polarised and
so Polaroid™ sunglasses can preferentially block this ‘glare’
o In 3-D cinema shows the image seen by the left and right eyes
are polarised and the special glasses one wears can block ‘the
other’ polarisation.
Image from Nikon Web Site

http://www.microscopyu.com/articles/polarized/polarizedlightintro.html
18
You said…
 The song
mentioned
cosmic rays.
What are they?
How much do we
know about
them?
 Is the
electromagnetic
range different
for animals.
Michael said…
 Our first appreciation that some kind of radiation came from
‘the cosmos’ was when we first made radiation detectors.
Scientist put films on balloons and found that the was more
radiation the higher one went in the atmosphere. They called
this cosmic radiation. No one knew what it was.
 In the 1950s when these songs were made the general belief
was that the source of this radiation was very high frequency
electromagnetic waves.
 Now we understand most of this radiation is electrons,
positrons and protons. No one knows exactly where they come
from or what process accelerates particles to such
phenomenal energy. I wrote about this in a blog article a
couple of years ago: expect more news about this soon-ish as
a new satellite instrument is collecting data scientists have
only previously dreamed about.
http://protonsforbreakfast.wordpress.com/2011/05/16/cosmic-adventure/
By this I think you mean can animals see different ranges of
electromagnetic radiation. Yes. Insects which go out in the
daylight can see into the ultraviolet. Some animals which hunt
in the dark, such as snakes, can apparently sense infrared
radiation, as can insects such as can mosquitos.
What is the
 I am not sure by what you mean by ‘arrangement’. Can you
arrangement of
ask this again this week if you need more clarification?
antimatter and is it  A football has a mass of approximately 0.50 kg. If this
true that
annihilated with 0.5 kg of normal matter then the amount
antimatter that is
of energy released would be given by Einstein’s famous
the mass of a
Equation
football has a
E = m c2
potential to
destroy the
m = 0.5 + 0.5 = 1 kilogram
galaxy?
c = 300,000,000 metres per second
Thanks for the
great lecture.
E = 9 x 1016 joules




A nuclear explosion equivalent to 1 million tonnes of TNT
corresponds to the release of 4.184×1015 joules of energy.
So this is equivalent to the energy in about 20 very large
explosions. That’s a lot of energy but well short of that
required to ‘destroy the galaxy’
19
You said…
Michael said…
 Electron tree
 I don’t know how many Coulombs of electricity is used
downstairs. I see this
–but it is an extreme process. Andrew Hanson made a
on TV. How much
nice little video about how it’s done:
electricity is needed to
http://www.youtube.com/watch?v=W34a2zDfrTQ
charge the glass?
 41! That is so young – plenty of time to go ‘back to
 Absolutely love the
school’ in one way or another.
course. You make me
 Yes-ish. If the thing detecting the light is at a place
want to go back to
where destructive interference occurs then, yes, they
school at 41.
would see no light from the wire. But this won’t work
 Can interference be
at all wavelengths (colours) or for all directions.
used to make objects
 But this ‘cancellation’ by interference is used in
invisible?
‘sound-cancelling’ headphones. A microphone listens to
 Question 3: How does
the ambient sound and produces an extra sound which
the mirror on the moon
cancels the ambient sound in one place: the entrance
work? It reflects back
to your ear. The cancellation doesn’t take place
at the same angle no
everywhere – so in other places the sound becomes
matter which direction
louder – but that’s not where your ear is!
the light comes from.
 The ‘ray’ diagrams below show (a) how a normal mirror
works and (b) how two mirrors can be placed together
to reflect light back in the same direction in twodimensions. The mirror-on-the-moon is an analogous
three-dimensional arrangement of three mirrors at
right angles called a ‘corner cube’. You can try making
up the 2-mirror arrangement below with ordinary
mirrors. The results are shocking – when you lift your
right arm – the right arm on your reflection lifts up –
which is not what you are used to seeing.
(a)

(b)
I’m back. Ha ha ha. How do you find all
these links to songs and Dave’s
whizzing periodic table? Better than
last time. Amazing. See you next week!
20

Welcome back! I don’t know how I
found them originally – I guess it was
serendipity. Thank you for your kind
comments.