Lecture 2: Properties of Radiation Chapter 2 & 3 Petty http://rain.geos.ntnu.edu.tw/~chen/Resources/AtmPhy_w2.pdf

Properties of Radiation What is radiation? How it behaves at the most fundamental physical level? What conventions are used to classify it according to wavelength and other properties? How do we define the characteristics (e.g. intensity) that appear in quantitative descriptions of radiation and its interaction with the atmosphere

Properties of Radiation The Nature of Electromagnetic Radiation Electric and Magnetic fields (detectable at some distance from their source) F1=F2=Kc*q1q2 r2

Properties of Radiation The Nature of Electromagnetic Radiation - A changing magnetic field produces an electric field (this is the phenomenon of electromagnetic induction, the basis of operation for electrical generators, induction motors, and transformers). - Similarly, a changing electric field generates a magnetic field. - Because of this interdependence of the electric and magnetic fields, it makes sense to consider them as a single coherent entity—the electromagnetic field

Properties of Radiation The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. The sun, earth, and other bodies radiate electromagnetic energy of varying wavelengths. Electromagnetic energy passes through space at the speed of light in the form of sinusoidal waves. The wavelength is the distance from wavecrest to wavecrest (see the figure below)

Properties of Radiation Electromagnetic Waves EM wave propagate as rays will spread the wave’s energy over a larger area and weaken as it gets further away. - EM waves follow principle of superposition. - EM waves are “transverse” waves. The principle of superposition may be applied to waves whenever two (or more) waves travelling through the same medium at the same time. The waves pass through each other without being disturbed. The net displacement of the medium at any point in space or time, is simply the sum of the individual wave displacements. This is true of waves which are finite in length (wave pulses) or which are continuous sine waves. A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right angled) to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane. http://paws.kettering.edu/~drussell/Demos/superposition/superposition.html

WAVE NATURE OF LIGHT Blue: l = 400 nm Wavelength Red: l = 700 nm Blue: l = 400 nm Light is an electromagnetic wave. Different wavelengths in the visible spectrum are seen by the eye as different colors. Light is an electromagnetic wave. It consists of oscillating electric and magnetic fields traveling through space. This figure is a representation of the electric field of a light wave. The wavelength is the distance between two peaks on the wave. Different wavelengths are seen by the eye as different colors. Blue light has a wavelength of about 400 nm (0.4 mm). Red light has a wavelength of about 700 nm (0.7 mm). (1 nm = 10-9 m; 1 mm = 10-6 m) For about three hundred years there was a disagreement among scientists about the nature of light. Some believed light was a wave and others believed that light was a particle. Both factions were eventually able to support their positions with experimental evidence. It was not until the beginning of the twentieth century that the answer was discovered. When light travels through space, it acts like a wave. When light is emitted or absorbed by an atom, it acts like a particle. The “particle” of light is called a photon. It is not a material particle but rather a “quantum” that acts as though it is located in one place and has definite energy and momentum, like an ordinary particle. A photon is more accurately described as a packet of energy. The energy of a photon is related to the wavelength of the light. Shorter wavelength photons have more energy than long wavelength photons. Laser-Professionals.com 7

Properties of Radiation Electromagnetic Waves

Basic concepts and definitions Electromagnetic radiation: * Energy propagated in the form of an advancing electric and magnetic field disturbance. * Travels in wave form at the speed of light (c). * Wavelength ( ) is the physical distance between adjacent maxima or minima in the electric or magnetic field. unit: * Wave number ( ) is the number of wavelengths in a unit distance, i.e., unit: 1/cm * Frequency ( ) is the number of successive maxima or minima passing a fixed point in a unit of time, unit: cycle-per-second (cps) or 1/s

Frequency and wavelength Speed of light c Frequency (Hz) v =  Wavelength 1 hertz (Hz) = one cycle per second c = 3.0 x 108 ms-1 Weather Radar, 3GHz wavelength??

Frequency

Frequency Decomposition - What if electromagnetic disturbance is not a steady oscillating signal?

Frequency Decomposition Eq. (2.2): any EM fluctuation can be thought of as a composite of a number of different “pure” periodic fluctuation

Broadband vs. Monochromatic

Broadband vs. Monochromatic

ELECTROMAGNETIC SPECTRUM Blue Green Yellow Red Visible Radio Gamma Ray X-ray Ultraviolet Infrared Microwaves Radio Short Wavelength Long Wavelength The electromagnetic spectrum extends from gamma rays at the short wavelength end to radio waves at the long wavelength end. The visible spectrum is a narrow slice somewhere in the middle, with blue light at the short wavelength end and red light at the long wavelength end. The next shortest wavelength region from the visible is the ultraviolet. Ultraviolet light causes sunburn, skin cancer, and cataracts. The next longest wavelength region from the visible is the infrared. Infrared light is invisible to the eye but can be felt as heat. It can cause burns to the skin or eyes. Lasers operate in the ultraviolet, visible, and infrared regions of the spectrum. Lasers in each spectral region present unique safety issues. Lasers operate in the ultraviolet, visible, and infrared. Laser-Professionals.com 16

Major Spectral Bands --Visible Band

Relevant to remote sensing As a proportion of total solar irradiance: Total energy from 0 – 0.75μm                54%   – all energy up to infra-red Total energy from 0.39μm – 0.75μm      43%   – visible light only Total energy from 0 – 4μm                   99%   – all “shortwave” Total energy from 4-infinity                       1%   – all “longwave” Total energy from 13μm-infinity              0.03% – major 15μm CO2 band and above Terminology: >0.75μm is infra-red (slightly different conventions exist about the maximum value for visible light, but nothing substantial) 0-4μm is “shortwave” – a climate science convention referring to solar radiation 4μm-infinity is “longwave“- a climate science convention referring to terrestrial radiation

Answer:

Radiation properties Quantum description Wave description

Quantum Properties of Radiation

STIMULATED EMISSION Incident Photon Incident Photon Excited Atom Stimulated Photon same wavelength same direction in phase When energy is absorbed by an atom, some of the electrons in that atom move into larger, higher energy orbits. When energy is released by the atom, the electrons move to smaller orbits. The lowest energy state is called the ground state. This is when all the electrons are as close to the nucleus as possible. Higher energy states are called excited states. Excited atomic states are not stable. Excited atoms tend to release energy in the form of photons and drop to lower energy states. Ordinary light is produced by spontaneous emission as excited atoms drop to lower energy levels and release photons spontaneously. The result is light that is a mixture of many different wavelengths, is emitted in all directions, and has random phase relationships. Laser light is produced by stimulated emission when excited atoms are struck by photons in the laser beam. This stimulates the excited atoms to emit their photons before they are emitted randomly by spontaneous emission. The result is that each stimulated photon is identical to the stimulating photon. This means that all photons produced by stimulated emission have the same wavelength, travel in the same direction, and are in phase. Thus the stimulated emission process leads to the unique properties of laser light. Laser-Professionals.com 23

When energy is absorbed by an atom, some of the electrons in that atom move into larger, higher energy orbits. When energy is released by the atom, the electrons move to smaller orbits. The lowest energy state is called the ground state. This is when all the electrons are as close to the nucleus as possible. Higher energy states are called excited states. Excited atomic states are not stable. Excited atoms tend to release energy in the form of photons and drop to lower energy states. Ordinary light is produced by spontaneous emission as excited atoms drop to lower energy levels and release photons spontaneously. The result is light that is a mixture of many different wavelengths, is emitted in all directions, and has random phase relationships. Laser light is produced by stimulated emission when excited atoms are struck by photons in the laser beam. This stimulates the excited atoms to emit their photons before they are emitted randomly by spontaneous emission. The result is that each stimulated photon is identical to the stimulating photon. This means that all photons produced by stimulated emission have the same wavelength, travel in the same direction, and are in phase. Thus the stimulated emission process leads to the unique properties of laser light.

Flux and Intensity

Flux and Intensity

Flux

Intensity - Spherical Polar Coordinate Fig. 2.3

Solid angle The ratio of the area of the sphere intercepted by the cone to the square of the radius Units: Steradians (sr) What is the area cut out of a sphere by one steradian? What is the solid angle representing all directions at a point?

HW2: A meteorological satellite circles the earth at a height h above the earth’s surface. Let the radius of the earth be and show that the solid angle under which the earth is seen by the satellite sensor is

Solid angle and definition of steradian

Solid angle and definition of steradian

Chapter 3 Electromagnetic Spectrum

Blackbody radiation Examine relationships between temperature, wavelength and energy emitted Blackbody: A “perfect” emitter and absorber of radiation... does not exist

Measuring energy Radiant energy: Total energy emitted in all directions (J) Radiant flux: Total energy radiated in all directions per unit time (W = J/s) Irradiance (radiant flux density): Total energy radiated onto (or from) a unit area in a unit time (W m-2) Radiance: Irradiance within a given angle of observation (W m-2 sr-1) Spectral radiance: Radiance for range in 

Radiance Toward satellite Normal to surface Solid angle, measured in steradians (1 sphere = 4 sr = 12.57 sr)

(sometimes indicated as E*) Stefan-Boltzmann Law M BB = T 4 Total irradiance emitted by a blackbody (sometimes indicated as E*) Stefan-Boltzmann constant The amount of radiation emitted by a blackbody is proportional to the fourth power of its temperature Sun is 16 times hotter than Earth but gives off 160,000 times as much radiation

Planck’s Function Blackbody doesn't emit equal amounts of radiation at all wavelengths Most of the energy is radiated within a relatively narrow band of wavelengths. The exact amount of energy emitted at a particular wavelength lambda is given by the Planck function:

Planck’s function c1-5 B  (T) = exp (c2 / T ) -1 First radiation constant Wavelength of radiation c1-5 B  (T) = exp (c2 / T ) -1 Absolute temperature Second radiation constant Irridance: Blackbody radiative flux for a single wavelength at temperature T (W m-2 m-1) Total amount of radiation emitted by a blackbody is a function of its temperature c1 = 1.19x10-16 W m-2 sr-1 c2 = 1.44x10-2 m K

Planck curve

Wein’s Displacement Law mT = 2897.9 m K Gives the wavelength of the maximum emission of a blackbody, which is inversely proportional to its temperature Earth @ 300K: ~10 m Sun @ 6000K: ~0.5 m

Intensity and Wavelength of Emitted Radiation : Earth and Sun

Solar Spectrum

window Atmosphere Window

Rayleigh-Jeans Approximation B (T) = (c1 / c2) -4 T When is this valid: 1. For temperatures encountered on Earth 2. For millimeter and centimeter wavelengths At microwave wavelengths, the amount of radiation emitted is directly proportional to T... not T4 B (T) TB = (c1 / c2) -4 Brightness temperature (TB) is often used for microwave and infrared satellite data, where it is called equivalent blackbody temperature. The brightness temperature is equal to the actual temperature times the emissivity.

Emissivity and Kirchoff’s Law  Actual irradiance by a non-blackbody at wavelength  Emittance: Often referred to as emissivity Emissivity is a function of the wavelength of radiation and the viewing angle and) is the ratio of energy radiated by the material to energy radiated by a black body at the same temperature absorbed/ incident Absorptivity (r , reflectivity; t , transmissivity)

Solar Constant The intensity of radiation from the Sun received at the top of the atmosphere Changes in solar constant may result in climatic variations http://www.space.com/scienceastronomy/071217-solar-cycle-24.html

CLOUD RADIATIVE FORCING Clouds can either warm or cool the climate depending on the cloud type Cooling By reflecting solar radiation back to space Particularly low clouds Global average short wave cooling is - 48 W m-2 Warming By acting as a greenhouse absorber and emitter of long wave radiation Particularly thin cirrus clouds Global average long wave warming is + 28 W m-2 Net Effect Global cooling of about - 20 W m-2 But what will the cloud feedback be with global