where
E = energy (in joules)
h = Planck’s constant (6.62 × 10⁻³⁴ joule-second)
f = the light’s frequency (in cycles per second = c/λ)
c = the velocity of light (300,000,000 meters per second)
λ = the wavelength (in meters)
Using the fact that an angstrom unit is 10⁻¹⁰ meter, the energy of a
5500 A photon could be calculated as
E = hf = hc/λ = (6.62 × 10⁻³⁴ × 3.00 × 10⁸)/(5.50 × 10⁻⁷)
= 3.61 × 10⁻¹⁹ joule = 2.2 electron volts
Comparing this result, 2.2 electron volts, with the energies required to
cause atomic ionization or molecular dissociation (an electron volt or
so), we see that it is in the right range to actuate direct conversion
devices based on such phenomena.