H A&S 222d
Introduction to Energy and Environment
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
m, that is millionths of
Light has many properties of a wave.
.an electromagnetic wave.
., particularly its
(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:
speed of light c = 3 x 10
meters/sec in a vacuum, but it is slower in glass or plastic or other
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
m, billionths of a m.).
We also described several triumphs of 1800s physics:
Maxwell’s equations describing electromagnetic (e.m.)
waves and unifying much of
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
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.
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