Light and the Electro-Magnetic spectrum
Koupelis - chapter 4
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What is the nature of light?
• Useful in the dark
• 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
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Hertz:
Experimental discovery oÍ radio waves, veritying Maxwell. 1888
Einstein:
Light interacts with matter as if it consists oÍ massless particles (photons),1905
Arastotle:
Light consists oÍ vibrations in the "aethei' that Íills all space
Newton:
Light consists oÍ tiny, fasÈ moving particles that cause the
"aethe/'to 1666
Young:
Light is a wave, as shown by interÍerence fringes in the double-slit experiment, 1801
Maxwell:
Light is an electromagnetic wave. 1860s
Planck:
Energy oÍ light waves is quantized and is proportional to Írequency, 1900
1500 AD Bohr:
Photons are generated or absorbed when electrons change energy levels in atoms, 1913
2(x,O AD 1000 AD
5OO AD
FIGURE 4-20 Over the years, explanations for the nature of light went back and forth between wave models and particle models. As we saw in the Advancing the Model box on pages I 12-113, today we talk about the wave-particle duality of light; it propagates through space as a wave but interacts with matter as a stream of particles.
Light : what is its nature?
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3
Light as an electro-magnetic wave
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Electric field
Magnetic field
A changing electric field induces a changing magnetic field,
and vice verse.
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Properties of light
• 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’.
• Photons with shorter wavelengths carry more energy.
(and electro-magnetic radiation in general)
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5
s aDout lo.) IIlIIluLcs LU [ratvcl d(luJl lrrc uldlllclcl ul ttlc
h's orbit (2 AU); Roemer's actual measurement was 22
FIGURE B4-2 Roemer's method of measuring the speed of light. The scale of the orbits of Earth and Jupiter has been changed to exaggerate the effect
Rotating toothed wheel Light 11 ,r.n\
source- b,,;F
t, 3 -sbKs
\ ï1u.Í-_//-
\ir' ''\ r
') Partially silvered mirror
Mirror
FIGURE B4-3 Fizeau's method of measuringthe speed oÍ light.
How to measure the speed of light?
Ole Roemer (1675) : orbital period of Io (1.76 days)
Result : 214.000 km/s (wrong, but almost correct)
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6
s aDout lo.) IIlIIluLcs LU [ratvcl d(luJl lrrc uldlllclcl ul ttlc h's orbit (2 AU); Roemer's actual measurement was 22
FIGURE B4-2 Roemer's method of measuring the speed of light. The scale of the orbits of Earth and Jupiter has been changed to exaggerate the effect
Rotating toothed wheel Light 11 ,r.n\
source- b,,;F t, 3 -sbKs
\ ï1u.Í-_//-
\ir'
''\ r
') Partially silvered mirror
Mirror
FIGURE B4-3 Fizeau's method of measuringthe speed oÍ light.
Armand Fizeau (1849) : double mirror with gear wheel
Result : 315.000 km/s (correct to 5%)
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How to measure the speed of light? 7
Light as a wave
Waves can interfere, so can light.
Light propagates through vacuum.
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Light as a wave
Frequency = speed wavelength Energy = constant x frequency
ν = c λ A few simple relevant formulas:
λ c
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E = h· ν = h·c / λ
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Light as a wave
Frequency = speed wavelength Energy = constant x frequency
ν = c λ A few simple relevant formulas:
λ c
8
E = h· ν = h·c / λ E = h· ν = h·c / λ E = h· ν = h·c / λ
E = h· ν = h·c / λ
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Light as a particle
CCD
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A CCD (Charge-Coupled Device)
counts photons, part of a digital camera! Also : E = hν = mc2 or : m = hν/c2
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‘sailing’ with radiation pressure
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The rainbow
White light consists of a rainbow of colours A ‘colour’ is light with a specific wavelength
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The spectrum
Newton (1666):
“Light consists of small, fast-moving particles”
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The Electro-Magnetic spectrum
13 FREQUENCY (Hz)
6 1 n 1 4 í n l 2
Ultraviolet
I
X-rays -
Visible Gamma rays
10-16 10-14 lO-12 10-í0 10-8 Wavelength
Radio
I T I Microwave py AM
10í 10-2 1 e 1 ( 1 0 6
500 600
Wavelength (nm)
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the electro-magnetic spectrum
Visible light is only a small part of the total electro-magnetic spectrum.
Typical wavelength of ‘visible’ electro-magnetic radiation:
500 nano-meter = 0.0005 millimeter (‘green’ light)
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Ultra-violet
(±10 – 390 nanometer)
more energetic than visible light It’s usually stopped by the ozone layer
Causes tanning, sun burn, skin cancer
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X-rays
(±0.03 – 3 nanometer)
Penetrates deep into materials.
medical applications screening luggage
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Gamma radiation
(shorter than ±0.03 nanometer)
Most energetic radiation Produced in radioactive decay
Very harmful to life
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Infrared
Less energetic than visible light
“heat radiation”
remote controls, security (±720 nanometer – 300 micrometer)
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Microwaves
(±300 micrometer – 1 centimeter)
(sub-) millimeter radiation micro-wave, radar
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Radiowaves
(±1 centimeter - kilometers)
Electro-magnetic radiation with the longest wavelengths;
radio, WiFi, navigation, radar etc
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Glowing objects
(black-body radiation)
Hot objects radiate from glowing red
to white hot metal, lava, stars, etc.
higher temperature lower 21
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Glowing objects
(black-body radiation)
Hot objects radiate, from glowing red to white hot metal, lava, stars, etc.
La Palma, Cumbre Vieja volcano, 1 Nov 2021 21
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Planck curve
glowing red
white hot
From colder to hotter:
-
Peak of the spectrum shifts to the blue (shorter wavelengths)-
Intensity of the radiation increases at all wavelengths23
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Wien’s law
wavelength of the peak (m) = 0.003 Temperature (K)
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The structure of atoms
discrete ‘orbits’ or energy levels of electrons in an atom
described by quantum-mechanics
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E E E
$ f l l | ' r ) O) @ l'- o N @
l l = 1
flLlURl 8'1.Í Íhe energy levels of the hydrogen atom (not drawn to scale). The levels are progressively closer in energy as they are Íarther from the nucleus. The len$h of each arrow corresponds to the energy involved in the process. An atom is ionizedwhen an electron absorbs enough energy and escapes.
Paschen 3- series - 2
Ground state r l = 1
fl0URË 81-0 Electrons that transit from the ground state to higher energy levels do so by absorbing photons of the wave- lengths shown. Electron jumps for the first three series of the hydrogen atom s spectrum are repÍesented here.
E (oc
@s E I(7)
s E o s
Io o o, . co
TU
Balmer series
I i l
-VisibrË rigtr 6o o o, q,c tiJ
Lyman series E E E
$ f l l | ' r ) O) @ l'- o N @
l l = 1
flLlURl 8'1.Í Íhe energy levels of the hydrogen atom (not drawn to scale). The levels are progressively closer in energy as they are Íarther from the nucleus. The len$h of each arrow corresponds to the energy involved in the process. An atom is ionizedwhen an electron absorbs enough energy and escapes.
Paschen 3- series - 2
Ground state r l = 1
fl0URË 81-0 Electrons that transit from the ground state to higher energy levels do so by absorbing photons of the wave- lengths shown. Electron jumps for the first three series of the hydrogen atom s spectrum are repÍesented here.
Ec (o
@s E I (7)s E o s
Io o o, . cTUo
Balmer series
I i l
-VisibrË rigtr o 6 o o, q, c tiJ
Lyman series
discrete energy levels with regularity Hydrogen-atom emission, absorption, ionization
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Absorption
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Emission & Absorption
emission lines
absorption lines 28
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Transmission
Reflection or scattering cloud
Absorption & Reflection
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spectra of astronomical objects
emission lines absorption lines
Solar spectrum Planck curve
continuum
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Information from the spectrum
• The continuum informs us about:
➢ the temperature of an object (Wien’s law)
• Emission & Absorption lines inform us about:
➢ which elements an object is made of.
➢ the concentration & temperature of these elements
Each element has a ‘finger print’
emission lines continuum
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Doppler effect
higher pitched sound lower pitched sound
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Doppler effect
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Object moves away from us : red shift Object does not move
We measure the Doppler shift, and thus velocities, using Emission and Absorption lines
Object moves towards us : blue shift golflengte ➞
Doppler effect
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the atmosphere
protects us against harmful radiation from
the Universe
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Atmospheric ‘windows’
The ‘optical’ window
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The “ - law”
R2 1
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