Light and the
Electro-Magnetic spectrum
Properties of light
(and electro-magnetic radiation in general)
• Light behaves like a wave (like sound)
!
• Light behaves like a particle: photons
!
• Light can propagate in vacuum
!
• Light moves at the maximum speed (usually indicated by c)
In vacuum: 299.792,458 km/s (roughly 1.08 billion km/hr)
In other media (air or glass) this speed is slightly lower.
!
• Light transports energy; each photon is an energy ‘package’.
!
Koupelis - chapter 4
• Photons with shorter wavelengths carry more energy.
1
3
4
lo.) IIlIIluLcs LU [ratvcl d(luJl
How
to measure the speedlrrcofuldlllclcl
light?
s aDout
ul ttlc
h's orbit (2 AU); Roemer'sactualmeasurementwas 22
What is the nature of light?
Ole Roemer (1675) : orbital period of Io (1.76 days)
• Useful in the dark
Ligh
!
sou
• A kind of electro-magnetic wave, carrying energy
!
• The only messenger from the Universe
\
available to astronomers
(except meteorites, Moon rocks, cosmic rays, gravity waves)
1
2
Result
: 214.000
km/s (wrong,
butmeasuring
almost correct)
the
method
of
FIGURE
B4-2Roemer's
speedof light.Thescaleoftheorbitsof Earthand
theeffect
to exaggerate
Jupiterhasbeenchanged
4
5
FIGURE
B4-3 F
How to measure the speed of light?
Light : what is its nature?
Armand Fizeau (1849) : double mirror with gear wheel
Arastotle:
Lightconsists
oÍ vibrations
in the "aethei'
thatÍillsall
space
Newton:
Lightconsists
oÍ tiny, fasÈ
movingparticles
thatcausethe
"aethe/'to
1666
Hertz:
Experimental
discoveryoÍ
Young:
radiowaves,
Lightis a wave,
veritying
as shownby
Maxwell.1888
interÍerence Maxwell:
fringesin the
Lightis an
double-slit
electromagnetic
experiment, wave.1860s
1801
Einstein:
ul ttlc
lrrc uldlllclcl
s aDout lo.) IIlIIluLcs LU [ratvcl d(luJl
Lightinteracts
(2 AU); Roemer'sactualmeasurementwas 22
h's orbit
as
with matter
if it consistsoÍ
masslessparticles
(photons),1905
Planck:
EnergyoÍ light
wavesis
quantizedand
is proportional
to Írequency,
1900
Bohr:
Photonsare
generatedor
absorbedwhen
electrons
changeenergy
levelsin atoms,
1913
Mirror
Rotating
toothedwheel
Light
11
source-
\
1000AD
5OOAD
1500AD
2(x,OAD
for the natureof lightwentbackand forth betweenwavemodelsand particle
4-20 Overthe years,explanations
FIGURE
dualityof
models.As we sawin the Advancingthe Modelboxon pagesI 12-113,todaywe talk aboutthe wave-particle
throughspaceas a wavebut interactswith matteras a streamof particles.
light;it propagates
the
method
of measuring
FIGURE
B4-2Roemer's
speedof light.Thescaleoftheorbitsof Earthand
theeffect
to exaggerate
Jupiterhasbeenchanged
,r.n\
b,,;F
t,ï1u.Í-_//3
-sbKs
\ir'
''\
r
')
Partially
silvered
mirror
speedoÍ light.
methodof measuringthe
FIGURE
B4-3 Fizeau's
2
3
Result : 315.000 km/s (correct to 5%)
6
5
Light as a wave
CCD
Light as a particle
Light propagates
through vacuum.
A CCD (Charge-Coupled Device) counts photons.
part of a digital camera!
Waves can interfere, so can light.
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7
8
10
Light as a wave
‘sailing’ with radiation pressure
λ
c
A few simple relevant formulas:
Frequency =
speed
wavelength
ν=
Energy = constant x frequency
c
λ
E = h· ν
7
8
9
11
Light as a wave
The rainbow
λ
c
A few simple relevant formulas:
Frequency =
speed
wavelength
ν=
Energy = constant x frequency
c
λ
E = h· ν
7
9
White light consists of a rainbow of colours
A ‘colour’ is light with a specific wavelength
12
10
The spectrum
Ultra-violet
(±10 – 390 nanometer)
Newton (1666):
“Light consists of small,
fast-moving particles”
more energetic than visible light
!
It’s usually stopped by the ozone layer
!
Causes tanning, sun burn, skin cancer
14
11
13
16
The Electro-Magnetic spectrum
X-rays
(Hz)
FREQUENCY
6
1n14
ínl2
(±0.03 – 3 nanometer)
Ultraviolet
Radio
I
X-rays
ITI
Visible
-
Microwavepy
Gamma rays
10-16 10-14
lO-12 10-í0
10í
10-8
10-2
AM
1e
1(
Penetrates deep into materials.
medical applications
screening luggage
106
Wavelength
600
500
Wavelength(nm)
12
14
15
17
the electro-magnetic spectrum
Gamma radiation
Visible light is only a small part of the total electro-magnetic spectrum.
(shorter than ±0.03 nanometer)
Most energetic radiation
Produced in radioactive decay
Very harmful to life
Typical wavelength of ‘visible’ electro-magnetic radiation:
!
500 nano-meter = 0.0005 millimeter
(‘green’ light)
13
15
16
18
Glowing objects
Infrared
(black-body radiation)
(±720 nanometer –
300 micrometer)
Hot objects radiate
!
from glowing red
to white hot
Less energetic than visible light
“heat radiation”
remote controls, security
!
metal, stars, etc.
17
20
19
22
Microwaves
white hot
glowing red
(±300 micrometer – 1 centimeter)
(sub-) millimeter radiation
micro-wave, radar
18
21
20
23
Planck curve
Radiowaves
From colder to hotter:
(±1 centimeter - kilometers)
Electro-magnetic radiation with the longest wavelengths;
radio, WiFi,
navigation, radar etc
!
-
Peak of the spectrum
shifts to the blue (shorter wavelengths)
-
Intensity of the
radiation increases
roodgoeiend
witheet
19
21
22
24
Absorption
Wien’s law
wavelength of the peak (m) =
0.003
Temperature (K)
26
23
25
28
Emission & Absorption
The structure of atoms
discrete ‘orbits’ or energy levels of electrons in an atom
emission lines
described by quantum-mechanics
absorption lines
24
26
27
29
discrete energy levels with regularity
emission, absorption, ionization
Hydrogen-atom
E E E
c
o I (o
@
s s s
(7)
EEE
EEE
$fll|'r)
O) @ l'oN@
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3-
3
E
E E - Paschen
Paschen
c
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series
o series
I
@
-2
- 2s s s
(7)
Balmerseries
Balmerseries
I
o
o
I
o
o
.c
o,
o
TU
ll=1
.c
Iil
Iil
-VisibrË -VisibrË
rigtr
o,
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rigtr
o
o
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o
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o
o,
o,
q,
c
tiJ
q,
c
tiJ
GroundstateGroundstate
rl= 1
LymanseriesLymanseries
ll=1
(not
atom(not
flLlURl
levelsof
the hydrogen
8'1.Í
Íhe
atom
flLlURl8'1.Í Íhe
levels
ofenergy
the hydrogen
energy
progressively
closerin energy
Thelevelsare
in energy
to scale).
closer
drawn
Thelevels
areprogressively
drawnto scale).
len$hof eacharrow
Thearrow
nucleus.
from
the
len$h
The
of each
as they
nucleus.
fromare
theÍarther
as theyareÍarther
process.
An atomis
in the
involved
is
to the energy
An
atom
corresponds
in the process.
to the
energyinvolved
corresponds
energyand escapes.
enough
absorbs
escapes.
an electron
energyand
enough
absorbs
ionizedwhen
anionizedwhen
electron
27
rl= 1
ground
from
the
to groundstateto
fl0URËthat
81-0transit
Electrons
state
fromthat
thetransit
fl0URË81-0 Electrons
photonsof the wavephotons
levelsdo so
byabsorbing
higher
of the wavedoenergy
so byabsorbing
higherenergylevels
jumps
jumpsfor
for
threeseriesof the
thefirst
Electron
series
of the
shown.
three
thefirst
Electron
lengthsshown.lengths
here.
repÍesented
atom
spectrumare
here.
aresrepÍesented
atomshydrogen
spectrum
hydrogen
25
28
30
Doppler effect
Absorption & Reflection
cloud
Transmission
higher pitched sound
lower pitched sound
Reflection
29
32
31
34
spectra of astronomical objects
Doppler effect
Planck curve
Solar spectrum
continuum
emission lines
absorption lines
30
33
32
35
Information from the spectrum
Doppler effect
• The continuum informs us about:
➢ the temperature of an object (Wien’s law)
We measure the Doppler shift, and thus velocities,
using Emission and Absorption lines
• Emission- & Absorption lines inform us about:
➢ which elements an object is made of.
Object does not move
➢ the concentration & temperature of these elements
emission lines
continuum
golflengte ➞
Object moves away from us : red shift
Object moves towards us
Each element has a ‘finger print’
33
: blue shift
34
31
36
the atmosphere
protects us against
harmful radiation from
the Universe
35
37
Atmospheric ‘windows’
The ‘optical’ window
36
38
The “
1
R
2
- law”
37
39