41 H → 2 He + 2+1e + energy (6.2.1)
(^40)
Electromagnetic Radiation
The space between the sun and Earth is mostly just that — empty space. So how
does the enormous amount of energy that the sun generates get carried through space
to Earth? It does so as electromagnetic radiation, which includes ultraviolet radiation,
visible light, infrared radiation, microwaves, and radio waves. Most of the energy that
actually reaches Earth’s surface does so as visible light and infrared radiation.
All electromagnetic radiation moves at the same speed through the vacuum of
space, a very fast 3.00 × 10^8 m/s (meters per second). As shown in Figure 6.3, the waves
of electromagnetic radiation have characteristics of wavelength (λ, Greek lambda),
amplitude, and frequency (ν, Greek “nu”). Wavelength and frequency are related by
the equation,
ν λ = c (6.2.2)
where ν is in units of cycles per second (s-^1 , a unit called the hertz, Hz), λ is in meters
(m), and c is the speed of light in m s-^1. The wavelength is the distance required for one
complete cycle and ν is the number of cycles per unit time.
Amplitude
Wavelength
Shorter wavelength,
higher frequency,
greater energy
Figure 6.3. Important characteristics of electromagnetic radiation.
Energy is associated with electromagnetic radiation. In addition to its wave character,
such radiation can be regarded as particles or quanta. According to the quantum theory
of electromagnetic radiation, electromagnetic radiation can be absorbed or emitted only
in discrete quanta, also called photons. A specific energy, E, is associated with each
photon and is related to the frequency, ν, of the associated electromagnetic radiation by
E = h ν (6.2.3)
where h is Planck’s constant, 6.63 × 10 -^34 J-s (joule × second). An important consequence
of this relationship is that the higher the frequency (the shorter the wavelength) of
electromagnetic radiation, the more energetic the photon associated with it.
There are some important environmental consequences associated with the energy
of photons, as well as some implications for green chemistry. As noted above, the sun’s
energy reaches Earth largely in the visible wavelength region, a very narrow band of
138 Green Chemistry, 2nd ed