Lecture 2: Properties of Radiation

1. Lecture 2: Properties of Radiation Chapter 2 & 3 Petty

2. 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

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

4. 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

5. 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)

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

7. l Wavelength Blue:l = 400 nm Red:l = 700 nm WAVE NATURE OF LIGHT Light is an electromagnetic wave. Different wavelengths in the visible spectrum are seen by the eye as different colors. Laser-Professionals.com

8. Properties of Radiation • Electromagnetic Waves

9. 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

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

11. Frequency

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

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

14. Broadband vs. Monochromatic

15. Broadband vs. Monochromatic

16. ELECTROMAGNETIC SPECTRUM Blue Green Yellow Red Visible Radio Gamma Ray X-ray Ultraviolet Infrared Microwaves Radio Short Wavelength Long Wavelength Lasers operate in the ultraviolet, visible, and infrared. Laser-Professionals.com

17. Major Spectral Bands --Visible Band

18. 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

20. Answer:

21. Radiation properties • Quantum description • Wave description

22. Quantum Properties of Radiation

23. STIMULATED EMISSION Incident Photon Incident Photon Excited Atom Stimulated Photon same wavelength same direction in phase Laser-Professionals.com

24. 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.

25. Flux and Intensity

26. Flux and Intensity

27. Flux

28. Intensity • - Spherical Polar Coordinate Fig. 2.3

29. 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?

30. 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

31. Solid angle and definition of steradian

32. Solid angle and definition of steradian

33. Chapter 3 Electromagnetic Spectrum

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

35. 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 

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

37. 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

38. 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:

39. Planck’s function 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

40. Planck curve

41. 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

42. Intensity and Wavelength of Emitted Radiation : Earth and Sun

43. Solar Spectrum

44. window Atmosphere Window

46. 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.

47. 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)

48. 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

49. 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