Energy and Human Ambitions on a Finite Planet, 2021a

5 <strong>Energy</strong> <strong>and</strong> Power Units 79
5.10 Light <strong>Energy</strong>
Light energy <strong>and</strong> its spectrum will be explored more extensively in
Chapter 13, but the main concepts are covered here for completeness.
Light can be used to describe any part of the electromagnetic spectrum,
from radio waves <strong>and</strong> microwaves, through infrared, visible, ultraviolet,
<strong>and</strong> on to X-rays <strong>and</strong> gamma rays. Like atoms, light is “quantized”
into smallest indivisible units—in this case particles called photons. An
individual photon’s energy is characteristic of its wavelength, λ (Greek
lambda), or frequency, ν (Greek nu). 38
Definition 5.10.1 The energy of a photon is given by
38: The two are related by the speed of
light, c, via λν c.
E hν hc
λ , (5.4)
where h 6.626 × 10 −34 J · s is Planck’s constant <strong>and</strong> c ≈ 3.0 × 10 8 m/s is
the speed of light.
Example 5.10.1 Visible light has a wavelength of 0.4–0.7 μm, 39 corresponding
to 2.8–5.0×10 −19 J for each photon.
39: A micron (μm, or micrometer) is another
way to say 10 −6 m.
We also routinely express photon energy in electron-volts (eV) according
to Definition 5.10.2.
Definition 5.10.2 Given the wavelength in microns (μm), the energy of a
photon in eV units is
E eV 1.24 eV. (5.5)
λ(μm)
Example 5.10.2 The red-end of the visible spectrum, around 0.7 μm,
corresponds to photon energies around 1.8 eV, while the blue-end,
around 0.4 μm, corresponds to 3.1 eV.
5.11 Upshot on Units
Every chapter has an upshot, usually distilling key lessons from the
chapter or offering final thoughts. Such a treatment is not necessary
here, although we could reinforce the idea that energy can always be
expressed in Joules, or converted into any of the units described in
the chapter. Also critical is the notion that energy is conserved—only
exchanging from one form to another but never truly disappearing or
coming from nowhere.
Students may wish to see a master table of conversions between all
the units discussed—<strong>and</strong> what a glorious table this would be! But it is
intentionally left out for three reasons:
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.