Direct Energy, 2018a

6 PHOTOVOLTAICS 103
The human eye can sense light from approximately λ = 400 nm to
λ = 650 nm. Using the expressions above, we can calculate in dierent units
the energy range over which the human eye can respond. An individual
red photon with λ = 650 nm has energy
E red =3.056 · 10 −19 J =1.908 eV =1.538 · 10 4 cm −1 (6.7)
in the dierent units. Similarly, an individual blue photon with λ = 400
nm has energy
E blue =4.966 · 10 −19 J =3.100 eV =2.500 · 10 4 cm −1 . (6.8)
We can calculate the energy of individual photons of electromagnetic
radiation at radio frequencies, at microwave frequencies, or in other frequency
ranges too. For example, the radio station WEAX broadcasts with
a frequency f =88MHz. This corresponds to a wavelength of λ =3.407 m.
An individual photon at this frequency has energy
E =5.831 · 10 −26 J =3.640 · 10 −7 eV. (6.9)
As another example, wi- operates at frequencies near f =2.4 GHz which
corresponds to the wavelength λ =0.125 m. Each photon at this frequency
has energy
E =1.590 · 10 −24 J =9.927 · 10 −6 eV. (6.10)
Ultraviolet light has a wavelength slightly shorter than blue light. A photon
of ultraviolet light with wavelength λ = 350 nm, which corresponds to
frequency f =8.57 · 10 14 Hz, has energy
E =5.676 · 10 −19 J =3.543 eV. (6.11)
X-rays operate at wavelengths near λ =10 −10 m. An x-ray photon with
wavelength λ =10 −10 m has energy
E =1.986 · 10 −15 J =1.240 · 10 4 eV. (6.12)
Why do we talk about radio waves but not radio particles while we treat
light as both wave-like and particle-like? A person is around 1.5 to 2 m
tall. The wavelength of the radio station broadcast in the example above
was λ RF ≈ 3.4 m while the wavelength of blue light was λ blue light ≈ 400
nm. Both radio frequency and optical signals are electromagnetic radiation.
Both are well described by Maxwell's equations. Both have wave-like and
particle-like properties. Humans typically talk about the wave-like nature
of radio waves because they are on a scale we can measure with a meter