CIVL1180

CIVL1180
Monitoring Changing Climate From Space

Prof. Hui Su
Dept. of Civil and Environmental Engineering
1
18 September, 2023
Brief Review of Last Class
• Satellite orbits
• Components of a Remote Sensing Stream
• EM spectrum
• Frequency and wavelength: f = C/l
Credit: EO4IM program

2
Interaction between electromagnetic waves and matter
In general, a ray incident on an obstacle, or on the separation surface between
two different media, generates reflection, refraction and diffraction phenomena.
There are also absorption, scattering, and transmission.
3
Reflection
• Reflection is the change in direction of a wavefront at
an interface between two different media so that the wavefront
returns into the medium from which it originated.
The Law of Reflection (specular reflection)

• The incident ray, the reflected ray and the
normal to the reflection surface at the point
of the incidence lie in the same plane.
• The angle which the incident ray makes with
the normal is equal to the angle which the
reflected ray makes to the same normal.
• The reflected ray and the incident ray are on
the opposite sides of the normal.
𝜃! = 𝜃"
4
Specular vs. Diffuse Reflection
• Specular reflection, or regular reflection, is the mirror-like

reflection of waves, such as light, from a surface.
• Diffuse reflection: is the reflection of light from a surface such
that an incident ray is reflected at many angles, rather than at
just one angle as in the case of specular reflection.
5
Specular and Lambertian Reflection
Lambertian Reflection: The reflected radiation is equally distributed

over all angles, irrespective of the direction of the source.
6
Refraction
• Refraction is the change in direction of a wave passing from
one medium to another
Snell’s Law:
For a given pair of media, the ratio of the sines of the angle
of incidence 𝜃! and angle of refraction 𝜃" is equal to the
ratio of phase velocities (v1 / v2 ) in the two media, or
equivalently to the indices of refraction (n2 / n1) of the two
media:
𝒔𝒊𝒏𝜃# 𝒗𝟏 𝒏𝟐
= =
𝒔𝒊𝒏𝜃$ 𝒗𝟐 𝒏𝟏
𝑛# 𝜈# = 𝑛$ 𝜈$ = C constant
7
Special Cases
• Case 1: 𝜃! = 𝜃" = 0, no change of direction

• Case 2, 𝜃! > 0, n2 > n1, the ray will bend toward the local normal
• Case 3: 𝜃! > 0, n2 < n1, the ray will bend away from normal
n1
n2
n1
8
Total Internal Reflection Law
𝜃!"#$!
9
Diffraction
• Diffraction is the process by which a beam of light or other system of
waves is spread out as a result of passing through a narrow aperture or
across an edge, typically accompanied by interference between the wave
forms produced.
In this case, the wave also propagates in

the part that should be in the shade,
bypassing the obstacle.
10
Rainbow
• Several interesting optical phenomena occur when rays of light
refract and reflect as they encounter the surface of particles which
have size much larger than the wavelength.
• For example, when visible sunlight (λ < 0.7 μm) encounters the
surface of large ice cloud particles (> 50 μm) and raindrops (100
μm < r < 3 mm).
9/20/23 11
11
Ray Tracing (Geometric Optics)
Now let’s follow the path of a single incident ray after it
intercepts the drop:
1) A fraction of the energy in the ray will be reflected

upon its first encounter with the surface of the drop.
2) Whatever is not reflected is transmitted into the drop
and refracted to an angle Θt relative to local normal.
3) The above ray now encounters the back side of the
drop, where a small fraction is reflected internally and
reminder exists the drop again at an angle Θi relative
to the local normal that is responsible for the primary
rainbow.
4) The part the exits the drop after exactly two internal
reflections is responsible for the secondary rainbow.
12
12
Rainbow
èWhen a beam of sunlight enters the water drop at point A, the degree of refraction
of the different colors is variable.
èViolet light has higher frequency, and its refractivity is larger, therefore its degree of
refraction is also the greatest; red light has longer wavelength and has smallest
degree of refraction. Other lights fall between them.
èThe rainbow we saw is always violet on the inside and red on the outside.
èThe colors and width of a rainbow are affected by the size of the raindrops. The
bigger the raindrops, the narrower and more colorful the rainbow is. 13
13
Other Types of Interaction
• The Doppler effect occurs when the source, target or propagation

medium are in motion
• Transmission occurs when radiation passes through a substance

without a significant attenuation
* The ability of a medium to transit energy is measured as the
transmittance
t = transmitted radiation/incident radiation
• Fluorescence occurs when an object illuminated with an electromagnetic
radiation at a specific wavelength emits radiation at a different
wavelength.
14
The Doppler Effect
• Doppler found that light and sound waves are affected by movement
• If something is moving towards you emitting sound, it shortens the
wavelength of the sound wave and creates higher pitch
Question: If driver hears the frequency ω, then what about

observers 1 and 2 hear? Can you list more examples? 15
Applications of the Doppler Effect in Remote Sensing
• Doppler radar is a specific type of radar that uses the Doppler
effect to gather velocity data from the particles that are being
measured. For example, a Doppler radar transmits a signal
that gets reflected off raindrops within a storm.
16
Applications of the Doppler Effect in Remote Sensing
• Edwin Hubble's brilliant observation was that the red shift of galaxies was
directly proportional to the distance of the galaxy from earth. That meant
that things farther away from Earth were moving away faster. In other
words, the universe must be expanding. He announced his finding in 1929.
Hubble's name is most widely recognized for the Hubble Space Telescope,
which was named in his honor.
Hubble Telescope, mass 11,110 kg, size 13.2 m long Pillars of Creation 17
Flux Density, Absorptivity and Reflectivity
• Flux density (in short, flux): the total energy per unit time (or
power) per unit area transported by EM radiation through a
plane, expressed in units of watts per square meter.
• When radiation is incident on the earth’s surface, some fraction is
reflected, and the remainder is absorbed. The fraction absorbed is
called absorptivity (a), while the fraction reflected is the
reflectivity (r). They are dependent on wavelength (𝜆).
𝑎! + 𝑟! = 1
Reflected flux: 𝐹'," =𝑟' 𝐹',)
Absorbed flux: 𝐹',) − 𝐹'," =(1 − 𝑟' )𝐹',) = 𝑎' 𝐹',) 18

Examples of Reflectivity Spectra
Typical shortwave reflectivity spectra of surface types as a function of wavelength

(see Petty’s Book Fig. 5.2) 19
The Graybody Approximation
• The graybody approximation is when we
ignore the dependence of absorptivity on
exact wavelength and use a single average
absorptivity to represent the entire spectral
band.
• A common application of the graybody
approximation is to assign one constant
absorptivity asw to the entire shortwave or
solar band, and another constant absorptivity
alw to the longwave or thermal IR band.
• The complement of the shortwave absorptivity
is the shortwave reflectivity, rsw = 1 – asw, also
commonly known as the shortwave albedo.
• A graybody with absorptivity a = 1 is a perfect
absorber, also called a blackbody.
20

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Monitoring Changing Climate From Space

Credit: EO4IM program

The Law of Reflection (specular reflection)

• Specular reflection, or regular reflection, is the mirror-like

Lambertian Reflection: The reflected radiation is equally distributed

• Case 1: 𝜃! = 𝜃" = 0, no change of direction

In this case, the wave also propagates in

1) A fraction of the energy in the ray will be reflected

• The Doppler effect occurs when the source, target or propagation

• Transmission occurs when radiation passes through a substance

Question: If driver hears the frequency ω, then what about

Absorbed flux: 𝐹',) − 𝐹'," =(1 − 𝑟' )𝐹',) = 𝑎' 𝐹',) 18

Typical shortwave reflectivity spectra of surface types as a function of wavelength

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