lecture2-3-09 - 1 H A&S 222d Introduction to Energy and...

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1 H A&S 222d Introduction to Energy and Environment P.B.Rhines Spring 2009 Lectures 2 and 3 some changes appear in RED Notes on SOLAR RADIATION and the EARTH’S GREENHOUSE Sun’s radiation and its spectrum. We introduced the sun’s energy, and the spectrum of wavelengths that describe the radiation. The color of the light is related to its wavelength, and outside of the band of visible colors (0.39 – 0.78 μm, 10 -6 m, that is millionths of a meter ++ , Light has many properties of a wave. .an electromagnetic wave. ., particularly its wavelength λ (m), frequency f (Hertz, or cycles per second) and wave speed, c (m/sec). These are related in the usual sense that distance from one wave-peak to the next = speed x time it takes a wave-peak to recur, which translates here to wavelength = speed/frequency: λ = c/f. The speed of light c = 3 x 10 8 meters/sec in a vacuum, but it is slower in glass or plastic or other transparent media. Hence we have refraction, which bends light rays as they move, say, from air to glass, giving us lenses and prisms. The speed c varies with wavelength, so that white light, which combines all colors, is split into its rainbow of component waves. Of course it is very fast, light taking 8 minutes 20 sec to travel the 150 million km from the sun. But physicists have succeeded in slowing light to a walking speed in the lab! Figure: color of light as a function of its wavelength, in nanometers (10 -9 m, billionths of a m.). We also described several triumphs of 1800s physics: 1) Maxwell’s equations describing electromagnetic (e.m.) waves and unifying much of electricity, magnetism. They show how a changing electric field (like a moving electron) produces a magnetic field and a changing magnetic field produces an electric field. This is the basis for both electric motors and e.m. waves. The origin of electromagnetic waves is the vibration of charged particles making up atoms and molecules. When electrons orbiting about the nucleus jump from one energy level to the next they can absorb or emit e.m. radiation. More complex oscillations of the electron-proton (minus and plus) charges occur. A radio antenna, which might be 40m long to match with the wavelength of a 7 megaHertz (7 million cycle per second) wave is an orderly pattern of oscillating electrons in a conducting wire. 2) Planck’s law showing the shape of the spectrum (that is, the distribution of radiating energy among different wavelengths) of radiation from a hot planet, assumed a ‘perfect black
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2 body’ (that is, a body that radiates perfectly without any radiation being absorbed on the way out): If B( λ ) is the radiated power, as a function of wavelength 2 5 21 () exp( / ) 1 hc B hc kT λ λλ = h is known as Planck’s constant, k is known as Boltzmann’s constant, and c is the speed of light.
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lecture2-3-09 - 1 H A&S 222d Introduction to Energy and...

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