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1 Radiation & Greenhouse Gases Fall 2010, Lecture 4

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Radiation Radiation describes a process in which energetic particles or waves travel through a medium or space Radiation is often referred to as electromagnetic radiation (EMR) It comprises both electric and magnetic field components These components oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation 2
Classification of radiation Electromagnetic radiation is classified into several types according to the frequency, or alternatively its related property wavelength, of the wave 3

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Frequency Frequency is the number of occurrences of a repeating event per unit time. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency 4
Frequency Diagram 5 Sinusoidal waves of various frequencies Bottom waves have higher frequencies, and shorter periods, than those above Horizontal axis represents time

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Wavelength Wavelength of a sinusoidal wave is the spatial period of the wave, the distance over which the wave's shape repeats It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings 6
Wavelength Diagram Wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests, or troughs, or corresponding zero crossings as shown 7

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Relationship of Frequency and Wavelength λ = c/ν c is the speed of light in a vacuum, a fundamental constant of nature (cm/s) ν is the frequency, measured in cycles per second, now called hertz (cycles/sec) λ is the wavelength (cm/cycle) c = 29,979,245,800 cm/sec (29.979 x 10 9 cm/sec) 8
Radiation Classification Types Classification types include (in order of increasing frequency and decreasing wavelength) Radio waves Microwaves Infrared radiation Visible light Ultraviolet radiation X-rays Gamma rays 9

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10 Visible Light Visible light, as perceived by the human eye, lies between approximately 400 to 700 nanometers
What is a Greenhouse? A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls Greenhouses heat up because incoming visible solar radiation from the sun is absorbed by plants, soil, and other things inside the building 11

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Greenhouse Physics - 1 Glass is transparent to this radiation The warmed structures and plants inside the greenhouse re-radiate this energy in the infra-red, to which glass is partly opaque, and that energy is trapped inside the glasshouse 12
Greenhouse Physics - 2 Although there is some heat loss due to conduction, there is a net increase in energy (and therefore temperature) inside the greenhouse Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall 13

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14 Incoming Radiation Some trace gases are known as “greenhouse" gases because they function like the glass in a greenhouse
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