Lecture 2: Properties of Radiation

Chapter 2 & 3 Petty

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 (thisis the phenomenon of electromagnetic induction, thebasis of operation for electrical generators, inductionmotors, and transformers).- Similarly, a changing electric field generates a magneticfield.- Because of this interdependence of the electric andmagnetic fields, it makes sense to consider them as asingle 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)

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.

Properties of Radiation

http://paws.kettering.edu/~drussell/Demos/superposition/superposition.html

WAVE NATURE OF LIGHT

Light is an electromagnetic wave.

Different wavelengths in the visible spectrum are seen by the eye as different colors.

Wavelength

Red: = 700 nm

Blue: = 400 nm

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Properties of Radiation

• Electromagnetic Waves

Basic concepts and definitionsElectromagnetic 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

k/1k

/c

m

Frequency and wavelength

v = c

Frequency (Hz)

Wavelength

Speed of light

1 hertz (Hz) = one cycle per secondc = 3.0 x 108 ms-1 Weather Radar,

3GHzwavelength??

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

Radio

Long WavelengthShort Wavelength

Gamma Ray X-ray Ultraviolet Infrared Microwaves

Visible

ELECTROMAGNETIC SPECTRUM

Lasers operate in the ultraviolet, visible, and infrared.

Radio

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RedBlue YellowGreen

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

Excited Atom

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Stimulated Photon same wavelength same direction in phase

Incident Photon

• 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

• 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?

Solid angle

2/ r

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 angleunder which the earth is seen by the satellite sensor is

ea

)]/()2(1[2 2/12 hahha ee

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

Solid angle, measured in steradians(1 sphere = 4 sr = 12.57 sr)

Normalto surface

Stefan-Boltzmann Law

M BB = T 4

Total irradianceemitted 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

B (T) = c1-5

exp (c2 / T ) -1

Irridance:Blackbody radiative fluxfor a single wavelength at temperature T (W m-2 m-1)

Second radiation constantAbsolute temperature

First radiation constant Wavelength of radiation

Total amount of radiation emitted by a blackbody is a function of its temperaturec1 = 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 mSun @ 6000K: ~0.5 m

Intensity and Wavelength of Emitted Radiation : Earth and Sun

Solar Spectrum

Atmosphere Window

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 wavelengthsAt microwave wavelengths, the amount of radiation emitted is directly proportional to T... not T4

(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.

B (T)TB =

Emissivity and Kirchoff’s Law

Actual irradiance bya non-blackbodyat 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 FORCINGClouds can either warm or cool the climate

depending on the cloud typeCooling• By reflecting solar radiation back to

space• Particularly low clouds• Global average short wave cooling is - 48

W m-2Warming• 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-2Net Effect• Global cooling of about - 20 W m-2• But what will the cloud feedback be with

global