Lecture 3 Quantum Physics- Underlying Theory for Remote

Sensing

Professor Menglin S. Jin

Department of Meteorology

San Jose State University

diagram for remote sensing–solar radiation

Electromagnetic SpectrumRemote sensing relies on measurements in the

electromagnetic spectrum (except sonar)• Remote sensing of the ground from space

• Need to see through the atmosphere• The ground must have some feature of interest in that spectral region• Studying reflected light requires a spectral region where solar energydominates

• Radar approaches mean we need frequencies that we can generate • Also need to ensure that we are not affected by other radio sources

• Atmosphere should be transparent at the selected frequency • •

• Time of the measurements lead to selecting a specific band• Type of detector/sensor partially determined by the spectral bands

THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING

•Quanta, or photons (the energy packets first identified

by Einstein in 1905), are particles of pure energy

having zero mass at rest

•the demonstration by Max Planck in 1901, and more

specifically by Einstein in 1905, that electromagnetic

waves consist of individual packets of energy was

in essence a revival of Isaac Newton's

(in the 17th Century) proposed but then

discarded corpuscular theory of light

THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING

• light, and all other forms of EMR, behaves both as waves and as particles. This is the famous "wave-particle" duality enunciated by de Broglie, Heisenberg, Born, Schroedinger, and others mainly in the 1920s

THE QUANTUM PHYSICS UNDERLYING REMOTE SENSING

• How is EMR produced?

Essentially, EMR is generated when an electric charge is accelerated, or more generally, whenever the size and/or direction of the electric (E) or magnetic (H) field is varied with time at its source

The photon is the physical form of a quantum, the basic particle of energy studied in quantum mechanics (which deals with the physics of the very small, that is, particles and their behavior at atomic and subatomic levels).

The photon is also described as the messenger particle for EM force or as the smallest bundle of light.

This subatomic massless particle, which also does not carry an electric charge, comprises radiation emitted by matter when it is excited thermally, or by nuclear processes (fusion, fission), or by bombardment with other radiation (as well as by particle collisions).

It also can become involved as reflected or absorbed radiation.

Photons move at the speed of light: 299,792.46 km/sec (commonly rounded off to 300,000 km/sec or ~186,000 miles/sec).

Consult http://en.wikipedia.org/wiki/Photon for more details

PHOTON

Photon• Photon particles also move as waves and

hence, have a "dual" nature. These waves follow a pattern that can be described in terms of a sine (trigonometric) function, as shown in two dimensions in the figure below.

photon travels as an EM wave

• having two components, oscillating as sine waves mutually at right angles, one consisting of the varying electric field, the other the varying magnetic field

wave

the distance between two adjacent peaks on a wave is its wavelength λ

The total number of peaks (top of the individual up-down curve) that pass by a reference lookpoint in a second is that wave's frequency ν

(in units of cycles per second, whose SI version is Hertz [1 Hertz = 1/s-1])

ν ~ 1/λ

c (speed of light) = λν

Wave

• The wave amplitudes of the two fields are also coincident in time and are a measure of radiation intensity (brightness)

Planck's general equation

• E=hv • The amount of energy characterizing a photon is

determined using Planck's general equation • h is Planck's constant (6.6260... x 10-34 Joules-

sec), v (read as nu), representing frequency • A photon is said to be quantized, any given one

possesses a certain quantity of energy • Some other photon can have a different energy

value • Photons as quanta thus show a wide range of

discrete energies.

Planck's general equation

• Photons traveling at higher frequencies are therefore more energetic.

• If a material under excitation experiences a change in energy level from a higher level E2 to a lower level E1, we restate the above formula as:

where v has some discrete value determined by (v2 - v1)

Planck Equation

• Wavelength is the inverse of frequency

C= λv

V= c/λ

c is the constant that expresses the speed of light

•we can also write the Planck equation as

Class wake-up activity

• Calculate the wavelength of a quantum of radiation whose photon energy is 2.10 x 10-19 Joules; use 3 x 108 m/sec as the speed of light c

• A radio station broadcasts at 120 MHz (megahertz or a million cycles/sec); what is the corresponding wavelength in meters (hint: convert MHz to units of Hertz)

polychromatic vs. monochromatic

• A beam of radiation (such as from the Sun) is usually polychromatic (has photons of different energies)

• if only photons of one wavelength are involved the beam is monochromatic.

• the distribution of all photon energies over the range of observed frequencies is embodied in the term spectrum

photoelectric effect –measure photon energy level

• the discovery by Albert Einstein in 1905

•His experiments also revealed that regardless of the radiation intensity, photoelectrons are emitted only after a threshold frequency is exceeded

•for those higher than the threshold value (exceeding the work function) the numbers of photoelectrons released re proportional to the number of incident photons

• For more, read the Chapter on The Nature of Electromagnetic Radiation in the Manual of Remote Sensing, 2nd Ed

How these physics related to

remote sensing?

Electromagnetic Spectrum: Transmittance, Absorptance, and Reflectance

• Any beam of photons from some source passing through medium 1 (usually air) that impinges upon an object or target (medium 2) will experience one or more reactions that are summarized below:

Electromagnetic Spectrum: Transmittance, Absorptance, and Reflectance

• (1) Transmittance (τ) - some fraction (up to 100%) of the radiation penetrates into certain surface materials such as water and if the material is transparent and thin in one dimension, normally passes through, generally with some diminution.

• (2) Absorptance (α) - some radiation is absorbed through electron or molecular reactions within the medium ; a portion of this energy is then re-emitted, usually at longer wavelengths, and some of it remains and heats the target;

• (3) Reflectance (ρ) - some radiation (commonly 100%) reflects (moves away from the target) at specific angles and/or scatters away from the target at various angles, depending on the surface roughness and the angle of incidence of the rays.

the Law of Conservation of Energy: τ + α + ρ = 1.

When a remote sensing instrument has a line-of-sight with an object that is reflecting solar energy, then the instrument collects that reflected energy and records the observation.

Most remote sensing systems are designed to collect reflected radiation.

Important Concepts

• Another formulation of radiant intensity is given by the radiant flux per unit of solid angle ω (in steradians - a cone angle in which the unit is a radian or 57 degrees, 17 minutes, 44 seconds)

Important Concepts

• radiance is defined as the radiant flux per unit solid angle leaving an extended source (of area A) in a given direction per unit projected surface area in that direction

L = Watt · m-2 · sr-1

where the Watt term is the radiant flux

Radiance is loosely related to the concept of brightness as associated with luminous bodies

WRT remote Sensing

• What really measured by remote sensing detectors are radiances at different wavelengths leaving extended areas

Radiative Transfer

What happens to radiation (energy) as it travels from the

“target” (e.g., ground, cloud...) to the satellite’s sensor?

Processes:

transmissionreflectionscatteringabsorptionrefractiondispersiondiffraction

transmission

• the passage of electromagnetic radiation through a medium

• transmission is a part of every optical phenomena (otherwise, the phenomena would never have occurred in the first place!)

reflection

• the process whereby a surface of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached; the reflected radiation is at the same angle as the incident radiation.

Reflection from smooth surface

angle of incidence

angle ofreflection

light ray

Scattering

• The process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. No energy transformation results, only a change in the spatial distribution of the radiation.

Molecular scattering (or other particles)

Rayleigh Scattering vs Mie Scattering

• Rayleigh scattering (named after the British physicist Lord Rayleigh) is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules .

• the Rayleigh scattering intensity for a single particle is 1/λ4

• Scattering by particles similar to or larger than the wavelength of light is typically treated by the Mie scattering

Scattering from irregular surface

Absorption (attenuation)

• The process in which incident radiant energy is retained by a substance. – A further process always results from

absorption:• The irreversible conversion of the absorbed

radiation goes into some other form of energy (usually heat) within the absorbing medium.

substance (air, water, ice, smog, etc.)

incidentradiation

absorption

transmittedradiation

Atmosphere Window

window

Refraction

• The process in which the direction of energy propagation is changed as a result of: – A change in density within the propagation

medium, or– As energy passes through the interface

representing a density discontinuity between two media.

Refraction in two different media

less densemedium

more densemedium

Gradually changing medium

ray

wavefronts

low density

high density

Dispersion

• the process in which radiation is separated into its component wavelengths (colors).

The “classic” example

white light

prism

Diffraction

• The process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.

light

Solid object

shadowregion

Atmospheric Constituents:

empty spacemoleculesdust and pollutantssalt particlesvolcanic materialscloud dropletsrain dropsice crystals

Optical phenomena

process + atmosphericconstituent

opticalphenomena

atmosphericstructure

light

Atmospheric Structure

temperature gradient

humidity gradient

clouds

layers of stuff - pollutants, clouds

• The change of sky colour at sunset (red nearest the sun, blue furthest away) is caused by Rayleigh scattering by atmospheric gas particles which are much smaller than the wavelengths of visible light. The grey/white colour of the clouds is caused by Mie scattering by water droplets which are of a comparable size to the wavelengths of visible light.