7.2 Atmospheric Absorption of EM Waves
211
‘phase coherence’ will act similarly to a classical electromagnetic wave. 7 We knew
since Maxwell’s work in 1865 that light also carries momentum. (See Eq. (7.7).)
With Einstein’s work, we know that this momentum is imparted by photons, each
with momentum
p = E/c = hf/c = h/λ .
(7.11)
Note that the energy carried by an electromagnetic wave is always proportional
to the electric (or magnetic) field squared (refer to Eq. (7.4)). This energy can be
measured by the intensity of the wave, which is the energy arriving at a unit area per
unit time. In quantum theory, light intensity is proportional to the flux of photons
detected or emitted. Sunlight at ground level has an intensity of about 0.1 W/cm 2 ,
dangerous to the human eye if the light enters directly. Lasers, which we will discuss
later, have been made with light intensities of petawatts per square centimeter 8 in
very short pulses (and a total energy of 680 J). With a duration of a picosecond, a
10 12 W/cm 2 laser would deposit about 0.6 keV within the area of an atom, enough
to knock all the electrons off low-Z atoms. 9
7.2 Atmospheric Absorption of EM Waves
The Earth has sufficient gravity, a magnetic shield against solar wind, and the right
range of temperatures to hold a relatively dense humid nitrogen-rich atmosphere for
billions of years. (Mars is a counter example.) Our atmosphere serves as a partially
protective shield for life on the surface of the Earth. X-rays from the Sun and outer
space are largely blocked. Hard ultraviolet rays are attenuated, ever since plant life
produced oxygen in the atmosphere. In the stratosphere, ozone is produced by hard
ultraviolet light acting on oxygen molecules (O 2 ). Ozone has a strong absorption of
hard UV. Oxygen and ozone also have molecular absorption bands in the infrared
and microwave ranges. Water vapor (H 2 O) and carbon dioxide (CO 2 ) have a number
of infrared resonant absorption bands and strong microwave absorption bands.
Because of the absorption characteristics of the atmospheric gases, there is a
‘window’ in the electromagnetic absorption spectrum of the atmosphere in the
range of visible light. (The same ‘window’ exists between ultraviolet light and
infrared light in many homogeneous solids, such as glass.) Another ‘window’ exists
for radio waves. (See Fig. 7.3.) Except for the sharp nuclear resonant frequencies
7 A set of waves near one frequency will be coherent if the phase angle of each wave closely
matches those of all the other waves. The closer the phase angles are to each other, the more
uncertain will be the number of photons detected in the wave.
8 For example, the Lawrence Livermore Petawatt Laser.
9 The atomic number is Z, measuring the number of protons in the nucleus of an atom, and the
number of electrons in the neutral atom.
211
‘phase coherence’ will act similarly to a classical electromagnetic wave. 7 We knew
since Maxwell’s work in 1865 that light also carries momentum. (See Eq. (7.7).)
With Einstein’s work, we know that this momentum is imparted by photons, each
with momentum
p = E/c = hf/c = h/λ .
(7.11)
Note that the energy carried by an electromagnetic wave is always proportional
to the electric (or magnetic) field squared (refer to Eq. (7.4)). This energy can be
measured by the intensity of the wave, which is the energy arriving at a unit area per
unit time. In quantum theory, light intensity is proportional to the flux of photons
detected or emitted. Sunlight at ground level has an intensity of about 0.1 W/cm 2 ,
dangerous to the human eye if the light enters directly. Lasers, which we will discuss
later, have been made with light intensities of petawatts per square centimeter 8 in
very short pulses (and a total energy of 680 J). With a duration of a picosecond, a
10 12 W/cm 2 laser would deposit about 0.6 keV within the area of an atom, enough
to knock all the electrons off low-Z atoms. 9
7.2 Atmospheric Absorption of EM Waves
The Earth has sufficient gravity, a magnetic shield against solar wind, and the right
range of temperatures to hold a relatively dense humid nitrogen-rich atmosphere for
billions of years. (Mars is a counter example.) Our atmosphere serves as a partially
protective shield for life on the surface of the Earth. X-rays from the Sun and outer
space are largely blocked. Hard ultraviolet rays are attenuated, ever since plant life
produced oxygen in the atmosphere. In the stratosphere, ozone is produced by hard
ultraviolet light acting on oxygen molecules (O 2 ). Ozone has a strong absorption of
hard UV. Oxygen and ozone also have molecular absorption bands in the infrared
and microwave ranges. Water vapor (H 2 O) and carbon dioxide (CO 2 ) have a number
of infrared resonant absorption bands and strong microwave absorption bands.
Because of the absorption characteristics of the atmospheric gases, there is a
‘window’ in the electromagnetic absorption spectrum of the atmosphere in the
range of visible light. (The same ‘window’ exists between ultraviolet light and
infrared light in many homogeneous solids, such as glass.) Another ‘window’ exists
for radio waves. (See Fig. 7.3.) Except for the sharp nuclear resonant frequencies
7 A set of waves near one frequency will be coherent if the phase angle of each wave closely
matches those of all the other waves. The closer the phase angles are to each other, the more
uncertain will be the number of photons detected in the wave.
8 For example, the Lawrence Livermore Petawatt Laser.
9 The atomic number is Z, measuring the number of protons in the nucleus of an atom, and the
number of electrons in the neutral atom.