Download Electromagnetic Waves In this lecture Waves Speed of

In this lecture
Introduction
Photons
Electromagnetic spectrum
WaveWave-particle duality
Matter & Energy
Electromagnetic Waves
Waves
Speed of Electromagnetic Waves
Mechanical Waves
• Transmission via a medium
• Velocity of wave governed by mechanical
properties of medium
Mechanical waves
•
Transport energy by mechanical
displacement of particles in a medium
Electromagnetic waves
•
Electromagnetic Waves
Transport energy form through empty
space
–
• Requires no medium to transmit
• Velocity is constant
How is the energy transferred?
Electromagnetic Waves
Electromagnetic Waves
• A singlesingle-frequency
EM wave exhibits a
sinusoidal variation of
electric and magnetic
fields in space
• Magnetic variation is
perpendicular to
electric field
• EM waves transport energy through
λ
space
Electric
Field
Magnetic
Field
• Energy is stored in the propagating
electric and magnetic fields
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Photons
Electromagnetic Spectrum
• A photon is the smallest quantity of
• Frequency Range: 10 – 1024 Hz
• Wavelength Range: 106 – 10-16 m
• Can be pictured as a small bundle of
• Three regions relevant to medical
ANY type of electromagnetic radiation
energy travelling through space at the
speed of light
Electromagnetic Spectrum
Frequency
(Hz)
Wavelength
(m)
1010
1024
10-16
108
1022
10-14
X-ray
Imaging
106
1020
10-12
104
1018
10-10
Visual
Imaging
UV
102
1016
10-8
10
1014
1μm 10-6
Visible Light
IR
10-2
1012
10-4
10-4
1010
10-6
108
MR
Imaging
Frequency, Wavelength & Energy
Gamma rays
X-rays
1cm 10-2
μ-wave
Radio
10-8
106
10-10
104
0
1m 10
102
1km
104
10-12
102
106
• Photons are quanta of radiation, which
– Particles
• Electromagnetic Radiation is
characterised by frequency and
wavelength
• Radio: measured via oscillations of electrons in
conductors
– Quoted in Hz
• Visible light: Early experiments describe light
as a wave
– Quoted in meters
• X-rays produced using electric potential
Wave Particle Duality
have energy
1. X-radiation
2. Visible
3. Radiofrequency
Measurement of EM Radiation
Energy
(eV)
eV)
1nm
imaging
– Quoted in keV
Wave Particle Duality
• Photons interact with matter most easily
when the matter is approximately the
same size as the photon wavelength
• What is the size of particles in matter?
– Waves
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Wave Like Behaviour of EM Radiation
Wave Particle Duality
• Photons from different regions of the
spectrum are fundamentally the same
• Difference in frequency results
differences in the way photons interact
with matter
• Visible photons behave more like waves
• X-ray photons act more like particles
Point Source
Interference
fringes
Interference
• This effect
can also be
observed for
mechanical
waves
Wave Like Behaviour of EM Radiation
Point Source
Interference
fringes
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Particle Behaviour or EM Radiation
• X-rays are identified by their energy
(1 – 50MeV)
• Wavelength range: 10-10 – 10-12m
• Usually smaller than objects!
• Therefore usually behave as particles
Example:
Planck’s Quantum Theory
• X-ray photons are characterised by energy
• Planck developed relationship between energy
and frequency
E = hf
• h is Planck’
Planck’s constant 6.63 X 10-34 Js-1
What happens if current through bulb
is reduced until only ONE photon is
emitted at a time?
What is the frequency of a 100keV
x-ray photon?
What happens if current through bulb
is reduced until only ONE photon is
emitted at a time?
What happens if current through bulb
is reduced until only ONE photon is
emitted at a time?
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What happens if current through bulb
is reduced until only ONE photon is
emitted at a time?
Matter & Energy
Classical Physics
• Matter can neither be created nor
destroyed
– Conservation of matter
• Energy can neither created nor
destroyed
– Conservation of energy
Matter & Energy
Example
• Planck & Einstein extended classical theories
PET scanner
When positronpositron-electron annihilation
occurs what are the energies of the two
photons produced?
• Matter can be transformed into Energy and
vice versa
E = mc 2
Summary
What are electromagnetic waves
Photons
Electromagnetic spectrum
WaveWave-particle duality
Matter & Energy
Practice Questions
PAM2011: Lecture 11 Problem Sheet
1. What is the energy in Joules of a 50keV x-ray photon?
2. What is the frequency of a 50keV x-ray photon?
3. What is the wavelength of a 50keV x-ray photon?
4. What is the mass equivalence of a 50keV X-ray photon?
5. Calculate the energy of a 400nm photon?
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