Photons as Energy Carriers
All our fossil fuels, such as coal and oil, owe their existence to the solar energy stream that has engulfed the earth for billions of years. The power in this stream amounts to about 1400 watts per square meter at the earth, nearly enough to supply an average home if all the energy were converted to electricity. The problem is to get the sun’s rays to yield up their energy with high efficiency.
The sun’s visible surface has a temperature around 6000°K. Any object heated to this temperature will radiate visible light mostly in the yellow-green portion of the spectrum (5500 A[11]). Our energy conversion device should be tuned to this wavelength.
The energy packets arriving from the sun are called photons. They travel, of course, at the speed of light, and each carries an amount of energy given by
E = hf = hc/λ
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.