FOR 435
Remote Sensing of Active-Fire and
Post-Fire Effects
Presentation 1-5
The Fundamentals of
Electromagnetic Radiation
Good Day!
The lecture is entitled the fundamentals of electromagnetic radiation.
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1.5.1 The History of Electromagnetic Radiation
The question of what light is made of has been asked by
people for centuries.
In the 1600s: Isaac Newton thought light = particles
This view of what light is changed in 1860
when James Clerk Maxwell presented his
‘Electromagnetic Theory’ of light.
In order to understand how we can use measurements of the reflection or emission
of electromagnetic radiation from surfaces to provide information about those
surfaces, we first need to understand more fully what electromagnetic radiation is
and what its properties are.
In the 1860s a physicist names James Clark Maxwell, who is shown in this
photograph, described Electromagnetic Radiation as a wave that travels through
space at the speed of light.
Prior to this light and other forms of electromagnetic radiation was thought to
consist of particles.
We will show in this lecture that Electromagnetic Radiation does exhibit the
properties of a wave.
However, we will also show that it also exhibits some properties that can not be
explained by it being a wave.
We will then discuss why these properties are important for remote sensing.
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1.5.2 Wave Terminology
Wavelength (λ) is the distance from one wave crest to the next. Typically
expressed in nanometers or micrometers.
Frequency (ν) is the number of crests passing a fixed point in a given period.
Typically expressed in hertz.
Amplitude is the height of each peak. Typically expressed in Watts/ meter2 /
μmeter.
c = λν
c = speed of light
Notice that in the schematic of an Electromagnetic wave it consists of two
oscillating waves at right angles to each other. These represent the electric and
magnetic waves.
Before we describe the properties of Electromagnetic Radiation and light we first
will briefly describe terminology of all waves, whether light, sound, etc
The wavelength is defined as the distance between two adjacent crests or two
adjacent troughs that exist on the same axis.
Therefore, in the case of an Electromagnetic wave shown in this figure we can show
the wavelength to be the distance between the 2nd and 3rd crests of the electric wave.
Although this equally could have been calculated using the crests or troughs of the
magnetic wave.The two commonly used units of wavelength are nanometer (10E-9)
and micrometers (10E-6), where 1 micrometer or micron is equal to 1000
nanometers.
The Frequency is defined as the number of crests passing a fixed point within a unit
time period and expressed in hertz.
Lastly the amplitude is defined as the height of each peak as measured from y=0.
Typical units of amplitude are
Watts/ meter2 / μmeter.
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1.5.3 Properties of Electromagnetic Radiation
Reflection
Specular reflection:
angle of incidence = angle of reflection
Diffuse (Lambertian) reflection:
light is scattered equally in all directions
The first property of light that we will consider is reflection.
In the case of a perfectly smooth reflective surface, such as we nearly achieve with
mirrors or with ice and snow, the angle of reflection will equal the angle of
incidence. This is called specular reflection, where specular is derived form the
Latin word for mirror. Examples of specular refection are common place, like when
you view yourself in the mirror, or use the mirror to glance around a corner.
In the case of a real surfaces, the light is scattered equally in all directions. This is
called diffuse or Lambertian reflection. If the light were not scattered in all
directions you would only be able to view objects if you were looking at them from
one single direction, which is clearly not always the case.
Importantly both waves and a stream of particles exhibit these properties.
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1.5.3 Properties of Electromagnetic Radiation
Refraction
Index of refraction (n):
ratio between the velocity of light in a vacuum (c) to its velocity in
the medium (v):
n
n’
n=c/v
θ
Snell’s law:
θ
θ’
n sin θ = n ' sin θ '
where n and n ' are the indices of refraction of the two media
The next property of light we will consider is refraction.
Refraction is the bending of light
rays at the contact between two
surfaces.
Importantly the ‘speed’ of light, c,
often quoted as 3 x ten power 8
meters per second, is more correctly
the speed of light in a vacuum. Light
always travels more slowly in any
other medium, essentially in a
vacuum there is nothing to slow it
down.
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1.5.3 Properties of Electromagnetic Radiation
Diffraction
The next property of light we will consider is diffraction.
Diffraction is a property of waves that allows them to be bent around a corner or
around obstacles. This is the same property that allows you to hear sounds around a
corner. An example of light diffraction is when you use a flashlight to make an
object cast a shadow, as is shown in this figure. If some of the light was not bend
around the object then the shadow would be perfectly black, which is not the case.
This property does not hold true for particles and thus provides evidence that light is
a wave.
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1.5.3 Properties of Electromagnetic Radiation
Interference
+
=
Constructive
Destructive
Light Source
We will now consider interference, which is another property of light that can not
be explained by the particle model.
If two waves identical in every form reach a single point in space the combined
wave will equal the sum of the two waves.
If two peaks arrive at the same time, you get a very large peak or if tw troughs are
large trough - This is an example of constructive interference as shown in the above
figure.
However, if a peak and trough arrive at the same time, they cancel each other out in
a process called destructive interference as shown in the figure.
The double slit experiment shows the interference of light. Essentially as in the
diffraction slide, when light reaches each gap it diffracts and light in seen from all
directions. When the peaks overlap bright patches are seen, but when the peaks and
troughs overlap dark regions are seen. This is simply the constructive and
destructive interference. In the case of particles, it is possible to explain the
constructive case as both sets of particles essentially creating a more intense particle
beam, but there is no way to explain the case of the particles canceling themselves
out. Again this is another compelling reason to believe that light is a wave rather
than a particle.
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1.5.3 Properties of Electromagnetic Radiation
Photoelectric Effect
Electrons
Light shining on clean
sodium metal in a
vacuum
The problems with this result if light is a wave:
Increasing the intensity of the light INCREASED the number of electrons
emitted but NOT their energy
Red Light does not cause any electrons to be emitted
Let us now consider a further property of light called the photoelectric effect.
This was an experiment conducted by Einstein who showed that when you shine
light on sodium in a vacuum you are able to produce electrons.
This experiment led to several problems of the wave model of light. Namely,
intensity of the light INCREASED the
number of electrons emitted but NOT their
energy.
-Increasing the
To explain - If light was a indeed wave, then
increasing its intensity would simply increase
its amplitude and we would thus expect the
same number of emitted electrons, except
that when hit by the light they would be forced
out of their orbits with a higher speed and
therefore a higher kinetic energy. This
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1.5.3 Properties of Electromagnetic Radiation
Properties of EM Radiation
Can be Explained by:
Wave
Particle
Reflection
Yes
Yes
Refraction
Yes
Yes
Diffraction
Yes
No
Interference
Yes
No
Photoelectric Effect
No
Yes
In summary we can see that light exhibits properties that can be explained by both
the particle and wave models and thus light it neither solely a particle of a wave.
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1.5.3 Properties of Electromagnetic Radiation
The Photon (Quantum) Model of Light:
Planck’s Hypothesis:
‘Light can only exist in discrete bundles with energy given by:
E = hv
Where, h = Planck’s constant = 6.626 E-34’
Light consists of bundles
of energy called photons
Electrons
This led to the development of what we now call the quantum model of light.
We assume that light is made up of bundles of energy that are called photons, where
each photon has the property of a wave.
This measure of photon’s energy explains why in Einstein’s experiment red light
failed to emit any electrons from the sodium. Essentially, the red light photons have
insufficient energy to successfully knock the sodium electrons off their orbits and
therefore nothing is detected. This does not change no matter how intense you make
the red light as this does not change the energy of the red photons. This is the same
principal behind detector arrays used in remote sensing systems, as different
substances are needed to detect light of different wavelengths.
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1.5.4 Using Photons in Remote Sensing
Waves:
Wavelengths may be split into
component bands or channels for
sensing and visualization, they can
further be related to specific biophysical
conditions within plants, soils, etc.
Particles:
Photons build up the electrical charge
on a sensor device/ccd and provide the
basis for differentiating patterns brightness
In summary, the nature of light being both a particle and a wave via the photon
model is very important for remote sensing.
This is because we are able to directly use properties of both models.
The wave model allows us to consider wavelengths and split these into bands or
channels, while the particle model allows us via the photoelectric effect to consider
light’s effect on sensors.
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